diff --git a/src/swf_typed/_activities.py b/src/swf_typed/_activities.py
index 0995033..70b4041 100644
--- a/src/swf_typed/_activities.py
+++ b/src/swf_typed/_activities.py
@@ -43,10 +43,10 @@ class ActivityInfo(_common.Deserialisable):
     created: datetime.datetime
     """Creation date."""
 
-    description: str = None
+    description: t.Union[str, None] = None
     """Activity description."""
 
-    deprecated: datetime.datetime = None
+    deprecated: t.Union[datetime.datetime, None] = None
     """Deprecation date."""
 
     @classmethod
@@ -105,7 +105,7 @@ def get_api_args(self):
 def delete_activity(
     activity: ActivityId,
     domain: str,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> None:
     """Delete a (deprecated/inactive) activity type.
 
@@ -122,7 +122,7 @@ def delete_activity(
 def deprecate_activity(
     activity: ActivityId,
     domain: str,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> None:
     """Deprecate (deactivate) an activity type.
 
@@ -139,7 +139,7 @@ def deprecate_activity(
 def describe_activity(
     activity: ActivityId,
     domain: str,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> ActivityDetails:
     """Describe an activity type.
 
@@ -162,9 +162,9 @@ def describe_activity(
 def list_activities(
     domain: str,
     deprecated: bool = False,
-    activity_filter: ActivityIdFilter = None,
+    activity_filter: t.Union[ActivityIdFilter, None] = None,
     reverse: bool = False,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> t.Generator[ActivityInfo, None, None]:
     """List activity types; retrieved semi-lazily.
 
@@ -197,9 +197,9 @@ def list_activities(
 def register_activity(
     activity: ActivityId,
     domain: str,
-    description: str = None,
-    default_task_configuration: DefaultTaskConfiguration = None,
-    client: "botocore.client.BaseClient" = None,
+    description: t.Union[str, None] = None,
+    default_task_configuration: t.Union[DefaultTaskConfiguration, None] = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> None:
     """Register a new activity type.
 
@@ -230,7 +230,7 @@ def register_activity(
 def undeprecate_activity(
     activity: ActivityId,
     domain: str,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> None:
     """Undeprecate (reactivate) an activity type.
 
diff --git a/src/swf_typed/_common.py b/src/swf_typed/_common.py
index ebfc38a..3183a1d 100644
--- a/src/swf_typed/_common.py
+++ b/src/swf_typed/_common.py
@@ -62,7 +62,7 @@ def get_api_args(self) -> t.Dict[str, t.Any]:
 
 
 def ensure_client(
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> "botocore.client.BaseClient":
     """Return or create SWF client."""
     if client:
diff --git a/src/swf_typed/_decisions.py b/src/swf_typed/_decisions.py
index e920e88..2e4b032 100644
--- a/src/swf_typed/_decisions.py
+++ b/src/swf_typed/_decisions.py
@@ -52,7 +52,7 @@ class CancelWorkflowExecutionDecision(Decision):
 
     type: t.ClassVar[str] = "CancelWorkflowExecution"
 
-    details: str = None
+    details: t.Union[str, None] = None
     """Execution cancellation details, usually for explanation."""
 
     def to_api(self):
@@ -70,7 +70,7 @@ class CompleteWorkflowExecutionDecision(Decision):
 
     type: t.ClassVar[str] = "CompleteWorkflowExecution"
 
-    execution_result: str = None
+    execution_result: t.Union[str, None] = None
     """Execution result."""
 
     def to_api(self):
@@ -88,16 +88,19 @@ class ContinueAsNewWorkflowExecutionDecision(Decision):
 
     type: t.ClassVar[str] = "ContinueAsNewWorkflowExecution"
 
-    execution_input: str = None
+    execution_input: t.Union[str, None] = None
     """Continuing execution input."""
 
-    workflow_version: str = None
+    workflow_version: t.Union[str, None] = None
     """Continuing execution workflow version."""
 
-    execution_configuration: "_executions.PartialExecutionConfiguration" = None
+    execution_configuration: t.Union[
+        "_executions.PartialExecutionConfiguration",
+        None,
+    ] = None
     """Continuing execution configuration overrides."""
 
-    tags: t.List[str] = None
+    tags: t.Union[t.List[str], None] = None
     """Continuing execution tags."""
 
     def to_api(self):
@@ -126,10 +129,10 @@ class FailWorkflowExecutionDecision(Decision):
 
     type: t.ClassVar[str] = "FailWorkflowExecution"
 
-    reason: str = None
+    reason: t.Union[str, None] = None
     """Execution failure reason, usually for classification."""
 
-    details: str = None
+    details: t.Union[str, None] = None
     """Execution failure details, usually for explanation."""
 
     def to_api(self):
@@ -154,7 +157,7 @@ class RecordMarkerDecision(Decision):
     marker_name: str
     """Marker name."""
 
-    details: str = None
+    details: t.Union[str, None] = None
     """Attached marker data."""
 
     def to_api(self):
@@ -191,7 +194,7 @@ class RequestCancelExternalWorkflowExecutionDecision(Decision):
     execution: t.Union["_executions.ExecutionId", "_executions.CurrentExecutionId"]
     """ID of execution to cancel."""
 
-    control: str = None
+    control: t.Union[str, None] = None
     """Message for future deciders."""
 
     def to_api(self):
@@ -215,13 +218,13 @@ class ScheduleActivityTaskDecision(Decision):
     task_id: str
     """Task ID."""
 
-    task_input: str = None
+    task_input: t.Union[str, None] = None
     """Task input."""
 
-    task_configuration: "_tasks.PartialTaskConfiguration" = None
+    task_configuration: t.Union["_tasks.PartialTaskConfiguration", None] = None
     """Task configuration overrides."""
 
-    control: str = None
+    control: t.Union[str, None] = None
     """Message for future deciders."""
 
     def to_api(self):
@@ -256,13 +259,13 @@ class ScheduleLambdaFunctionDecision(Decision):
     task_id: str
     """Task ID."""
 
-    task_input: str = None
+    task_input: t.Union[str, None] = None
     """Lambda function input."""
 
     task_timeout: datetime.timedelta = _common.unset
     """Lambda function invocation timeout."""
 
-    control: str = None
+    control: t.Union[str, None] = None
     """Message for future deciders."""
 
     def to_api(self):
@@ -298,10 +301,10 @@ class SignalExternalWorkflowExecutionDecision(Decision):
     signal: str
     """Signal name."""
 
-    signal_input: str = None
+    signal_input: t.Union[str, None] = None
     """Signal input."""
 
-    control: str = None
+    control: t.Union[str, None] = None
     """Message for future deciders."""
 
     def to_api(self):
@@ -328,16 +331,19 @@ class StartChildWorkflowExecutionDecision(Decision):
     execution: "_executions.CurrentExecutionId"
     """Child execution workflow-ID."""
 
-    execution_input: str = None
+    execution_input: t.Union[str, None] = None
     """Child execution input."""
 
-    execution_configuration: "_executions.PartialExecutionConfiguration" = None
+    execution_configuration: t.Union[
+        "_executions.PartialExecutionConfiguration",
+        None,
+    ] = None
     """Child execution configuration overrides."""
 
-    tags: t.List[str] = None
+    tags: t.Union[t.List[str], None] = None
     """Child execution tags"""
 
-    control: str = None
+    control: t.Union[str, None] = None
     """Message for future deciders."""
 
     def to_api(self):
@@ -374,7 +380,7 @@ class StartTimerDecision(Decision):
     timer_duration: datetime.timedelta
     """Timer duration."""
 
-    control: str = None
+    control: t.Union[str, None] = None
     """Message for future deciders."""
 
     def to_api(self):
@@ -406,7 +412,7 @@ class DecisionTask(_common.Deserialisable):
     decision_task_started_execution_history_event_id: int
     """History event ID for decision-task start."""
 
-    previous_decision_task_started_execution_history_event_id: int = None
+    previous_decision_task_started_execution_history_event_id: t.Union[int, None] = None
     """History event ID for previous decision-task start."""
 
     _execution_history_list: t.List["_history.Event"] = dataclasses.field(init=False)
@@ -416,7 +422,9 @@ def __post_init__(self):
 
     @classmethod
     def from_api(
-        cls, data, execution_history_iter: t.Iterable["_history.Event"] = None
+        cls,
+        data,
+        execution_history_iter: t.Union[t.Iterable["_history.Event"], None] = None,
     ) -> "DecisionTask":
         """Deserialise decision task from SWF API response data.
 
@@ -464,7 +472,7 @@ def execution_history(self) -> t.List["_history.Event"]:
 def get_number_of_pending_decision_tasks(
     task_list: str,
     domain: str,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> int:
     """Get the number of pending decision tasks.
 
@@ -491,9 +499,9 @@ def get_number_of_pending_decision_tasks(
 def request_decision_task(
     task_list: str,
     domain: str,
-    decider_identity: str = None,
-    no_tasks_callback: t.Callable[[], None] = None,
-    client: "botocore.client.BaseClient" = None,
+    decider_identity: t.Union[str, None] = None,
+    no_tasks_callback: t.Union[t.Callable[[], None], None] = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> DecisionTask:
     """Request (poll for) a decision task; blocks until task is received.
 
@@ -543,8 +551,8 @@ def iter_history() -> t.Generator["_history.Event", None, None]:
 def send_decisions(
     token: str,
     decisions: t.List[Decision],
-    context: str = None,
-    client: "botocore.client.BaseClient" = None,
+    context: t.Union[str, None] = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> None:
     """Make decisions for a workflow execution, completing decision task.
 
diff --git a/src/swf_typed/_domains.py b/src/swf_typed/_domains.py
index d0a491d..feb2392 100644
--- a/src/swf_typed/_domains.py
+++ b/src/swf_typed/_domains.py
@@ -24,7 +24,7 @@ class DomainInfo(_common.Deserialisable):
     arn: str
     """Domain ARN."""
 
-    description: str = None
+    description: t.Union[str, None] = None
     """Domain description."""
 
     @classmethod
@@ -72,7 +72,7 @@ def from_api(cls, data) -> "DomainDetails":
 
 def deprecate_domain(
     domain: str,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> None:
     """Deprecate (deactivate) a domain.
 
@@ -87,7 +87,7 @@ def deprecate_domain(
 
 def describe_domain(
     domain: str,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> DomainDetails:
     """Describe a domain.
 
@@ -107,7 +107,7 @@ def describe_domain(
 def list_domains(
     deprecated: bool = False,
     reverse: bool = False,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> t.Generator[DomainInfo, None, None]:
     """List domains; retrieved semi-lazily.
 
@@ -131,7 +131,7 @@ def list_domains(
 
 def get_domain_tags(
     domain_arn: str,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> t.Dict[str, str]:
     """Get a domain's tags.
 
@@ -151,9 +151,9 @@ def get_domain_tags(
 def register_domain(
     domain: str,
     configuration: DomainConfiguration,
-    description: str = None,
-    tags: t.Dict[str, str] = None,
-    client: "botocore.client.BaseClient" = None,
+    description: t.Union[str, None] = None,
+    tags: t.Union[t.Dict[str, str], None] = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> None:
     """Register a new domain.
 
@@ -182,7 +182,7 @@ def register_domain(
 def tag_domain(
     domain_arn: str,
     tags: t.Dict[str, str],
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> None:
     """Add tags to domain.
 
@@ -199,7 +199,7 @@ def tag_domain(
 
 def undeprecate_domain(
     domain: str,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> None:
     """Undeprecate (reactivate) a domain.
 
@@ -215,7 +215,7 @@ def undeprecate_domain(
 def untag_domain(
     domain_arn: str,
     tags: t.List[str],
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> None:
     """Remove tags from a domain.
 
diff --git a/src/swf_typed/_executions.py b/src/swf_typed/_executions.py
index 3fac67b..447710b 100644
--- a/src/swf_typed/_executions.py
+++ b/src/swf_typed/_executions.py
@@ -96,13 +96,13 @@ class ExecutionInfo(_common.Deserialisable):
     cancel_requested: bool
     """Execution cancellation has been requested."""
 
-    closed: datetime.datetime = None
+    closed: t.Union[datetime.datetime, None] = None
     """Execution end-date."""
 
-    parent: ExecutionId = None
+    parent: t.Union[ExecutionId, None] = None
     """Parent execution ID."""
 
-    tags: t.List[str] = None
+    tags: t.Union[t.List[str], None] = None
     """Execution tags."""
 
     @classmethod
@@ -153,10 +153,10 @@ class ExecutionConfiguration(_common.Deserialisable):
     child_execution_policy_on_termination: ChildExecutionTerminationPolicy
     """Child workflow execution ending policy on termination."""
 
-    decision_task_priority: int = None
+    decision_task_priority: t.Union[int, None] = None
     """Decision task priority."""
 
-    lambda_iam_role_arn: str = None
+    lambda_iam_role_arn: t.Union[str, None] = None
     """Execution IAM role ARN for Lambda invocations."""
 
     @classmethod
@@ -183,9 +183,12 @@ class PartialExecutionConfiguration(
 
     timeout: t.Union[datetime.timedelta, None] = _common.unset
     decision_task_timeout: t.Union[datetime.timedelta, None] = _common.unset
-    decision_task_list: str = None
-    decision_task_priority: int = None
-    child_execution_policy_on_termination: ChildExecutionTerminationPolicy = None
+    decision_task_list: t.Union[str, None] = None
+    decision_task_priority: t.Union[int, None] = None
+    child_execution_policy_on_termination: t.Union[
+        ChildExecutionTerminationPolicy,
+        None,
+    ] = None
 
     @classmethod
     def from_api(cls, data) -> "PartialExecutionConfiguration":
@@ -258,7 +261,7 @@ class ExecutionOpenCounts:
     child_executions: int
     """Number of started child executions."""
 
-    lambda_tasks: int = None
+    lambda_tasks: t.Union[int, None] = None
     """Number of scheduled/started Lambda invocations."""
 
     @classmethod
@@ -279,16 +282,16 @@ class ExecutionDetails:
     info: ExecutionInfo
     """Execution details."""
 
-    configuration: ExecutionConfiguration = None
+    configuration: t.Union[ExecutionConfiguration, None] = None
     """Execution configuration."""
 
-    n_open: ExecutionOpenCounts = None
+    n_open: t.Union[ExecutionOpenCounts, None] = None
     """Counts of open tasks/timers/children in execution."""
 
-    latest_activity_task_scheduled: datetime.datetime = None
+    latest_activity_task_scheduled: t.Union[datetime.datetime, None] = None
     """Most recent activity task's scheduled's date."""
 
-    latest_context: str = None
+    latest_context: t.Union[str, None] = None
     """Most recent decision's execution context."""
 
     @classmethod
@@ -319,7 +322,7 @@ class DateTimeFilter(_common.Serialisable):
     earliest: datetime.datetime
     """Earliest date."""
 
-    latest: datetime.datetime = None
+    latest: t.Union[datetime.datetime, None] = None
     """Latest date."""
 
     def to_api(self):
@@ -422,14 +425,19 @@ def _get_number_of_executions(
 
 def get_number_of_closed_executions(
     domain: str,
-    time_filter: t.Union[StartTimeExecutionFilter, CloseTimeExecutionFilter] = None,
+    time_filter: t.Union[
+        StartTimeExecutionFilter,
+        CloseTimeExecutionFilter,
+        None,
+    ] = None,
     property_filter: t.Union[
         IdExecutionFilter,
         WorkflowTypeExecutionFilter,
         TagExecutionFilter,
         CloseStatusExecutionFilter,
+        None,
     ] = None,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> int:
     """Get the number of closed workflow executions.
 
@@ -457,13 +465,14 @@ def get_number_of_closed_executions(
 
 def get_number_of_open_executions(
     domain: str,
-    started_filter: StartTimeExecutionFilter = None,
+    started_filter: t.Union[StartTimeExecutionFilter, None] = None,
     property_filter: t.Union[
         IdExecutionFilter,
         WorkflowTypeExecutionFilter,
         TagExecutionFilter,
+        None,
     ] = None,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> int:
     """Get the number of open workflow executions.
 
@@ -491,7 +500,7 @@ def get_number_of_open_executions(
 def describe_execution(
     execution: ExecutionId,
     domain: str,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> ExecutionDetails:
     """Describe a workflow execution.
 
@@ -513,15 +522,20 @@ def describe_execution(
 
 def list_closed_executions(
     domain: str,
-    time_filter: t.Union[StartTimeExecutionFilter, CloseTimeExecutionFilter] = None,
+    time_filter: t.Union[
+        StartTimeExecutionFilter,
+        CloseTimeExecutionFilter,
+        None,
+    ] = None,
     property_filter: t.Union[
         IdExecutionFilter,
         WorkflowTypeExecutionFilter,
         TagExecutionFilter,
         CloseStatusExecutionFilter,
+        None,
     ] = None,
     reverse: bool = False,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> t.Generator[ExecutionInfo, None, None]:
     """List closed workflow executions; retrieved semi-lazily.
 
@@ -554,14 +568,15 @@ def list_closed_executions(
 
 def list_open_executions(
     domain: str,
-    started_filter: StartTimeExecutionFilter = None,
+    started_filter: t.Union[StartTimeExecutionFilter, None] = None,
     property_filter: t.Union[
         IdExecutionFilter,
         WorkflowTypeExecutionFilter,
         TagExecutionFilter,
+        None,
     ] = None,
     reverse: bool = False,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> t.Generator[ExecutionInfo, None, None]:
     """List open workflow executions; retrieved semi-lazily.
 
@@ -595,7 +610,7 @@ def list_open_executions(
 def request_cancel_execution(
     execution: t.Union[CurrentExecutionId, ExecutionId],
     domain: str,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> None:
     """Request the cancellation of a workflow execution.
 
@@ -618,8 +633,8 @@ def signal_execution(
     execution: t.Union[CurrentExecutionId, ExecutionId],
     signal: str,
     domain: str,
-    input_: str = None,
-    client: "botocore.client.BaseClient" = None,
+    input_: t.Union[str, None] = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> None:
     """Send a signal to a workflow execution.
 
@@ -649,10 +664,10 @@ def start_execution(
     workflow: "_workflows.WorkflowId",
     execution: CurrentExecutionId,
     domain: str,
-    input: str = None,
-    configuration: PartialExecutionConfiguration = None,
-    tags: t.List[str] = None,
-    client: "botocore.client.BaseClient" = None,
+    input: t.Union[str, None] = None,
+    configuration: t.Union[PartialExecutionConfiguration, None] = None,
+    tags: t.Union[t.List[str], None] = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> ExecutionId:
     """Start a workflow execution.
 
@@ -689,10 +704,10 @@ def start_execution(
 def terminate_execution(
     execution: t.Union[CurrentExecutionId, ExecutionId],
     domain: str,
-    reason: str = None,
-    details: str = None,
-    child_execution_policy: ChildExecutionTerminationPolicy = None,
-    client: "botocore.client.BaseClient" = None,
+    reason: t.Union[str, None] = None,
+    details: t.Union[str, None] = None,
+    child_execution_policy: t.Union[ChildExecutionTerminationPolicy, None] = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> None:
     """Terminate (immediately close) a workflow execution.
 
diff --git a/src/swf_typed/_history.py b/src/swf_typed/_history.py
index 8f15da8..0f741c9 100644
--- a/src/swf_typed/_history.py
+++ b/src/swf_typed/_history.py
@@ -203,10 +203,10 @@ class ActivityTaskCancelledEvent(Event):
     task_started_event_id: int
     """Task start event ID."""
 
-    task_cancel_requested_event_id: int = None
+    task_cancel_requested_event_id: t.Union[int, None] = None
     """Task cancellation request event ID."""
 
-    details: str = None
+    details: t.Union[str, None] = None
     """Cancellation details, usually for explanation."""
 
     @classmethod
@@ -234,7 +234,7 @@ class ActivityTaskCompletedEvent(Event):
     task_started_event_id: int
     """Task start event ID."""
 
-    task_result: str = None
+    task_result: t.Union[str, None] = None
     """Task result."""
 
     @classmethod
@@ -261,10 +261,10 @@ class ActivityTaskFailedEvent(Event):
     task_started_event_id: int
     """Task start event ID."""
 
-    reason: str = None
+    reason: t.Union[str, None] = None
     """Failure reason, usually for classification."""
 
-    details: str = None
+    details: t.Union[str, None] = None
     """Failure details, usually for explanation."""
 
     @classmethod
@@ -298,10 +298,10 @@ class ActivityTaskScheduledEvent(Event):
     decision_event_id: int
     """Task schedule decision event ID."""
 
-    task_input: str = None
+    task_input: t.Union[str, None] = None
     """Task input."""
 
-    control: str = None
+    control: t.Union[str, None] = None
     """Message from task scheduling decider."""
 
     @classmethod
@@ -331,7 +331,7 @@ class ActivityTaskStartedEvent(Event):
     task_scheduled_event_id: int
     """Task schedule event ID."""
 
-    worker_identity: str = None
+    worker_identity: t.Union[str, None] = None
     """Identity of worker which acquired task."""
 
     @classmethod
@@ -360,7 +360,7 @@ class ActivityTaskTimedOutEvent(Event):
     task_started_event_id: int
     """Task start event ID."""
 
-    details: str = None
+    details: t.Union[str, None] = None
     """Most recent progress message from worker (see ``send_heartbeat``)."""
 
     @classmethod
@@ -446,7 +446,7 @@ class ChildWorkflowExecutionCancelledEvent(Event):
     execution_started_event_id: int
     """Child execution start event ID."""
 
-    details: str = None
+    details: t.Union[str, None] = None
     """Cancellation details, usually for explanation."""
 
     @classmethod
@@ -484,7 +484,7 @@ class ChildWorkflowExecutionCompletedEvent(Event):
     execution_started_event_id: int
     """Child execution start event ID."""
 
-    execution_result: str = None
+    execution_result: t.Union[str, None] = None
     """Child execution result."""
 
     @classmethod
@@ -522,10 +522,10 @@ class ChildWorkflowExecutionFailedEvent(Event):
     execution_started_event_id: int
     """Child execution start event ID."""
 
-    reason: str = None
+    reason: t.Union[str, None] = None
     """Failure reason, usually for classification."""
 
-    details: str = None
+    details: t.Union[str, None] = None
     """Failure details, usually for explanation."""
 
     @classmethod
@@ -710,7 +710,7 @@ class DecisionTaskCompletedEvent(Event):
     decision_task_started_event_id: int
     """Decision task start event ID."""
 
-    decision_context: str = None
+    decision_context: t.Union[str, None] = None
     """Context provided by decider, accessible when describing execution."""
 
     @classmethod
@@ -737,7 +737,7 @@ class DecisionTaskScheduledEvent(Event):
     decision_task_timeout: t.Union[datetime.timedelta, None] = _common.unset
     """Decision runtime timeout."""
 
-    decision_task_priority: int = None
+    decision_task_priority: t.Union[int, None] = None
     """Decision task priority."""
 
     @classmethod
@@ -764,7 +764,7 @@ class DecisionTaskStartedEvent(Event):
     decision_task_scheduled_event_id: int
     """Decision task schedule event ID."""
 
-    decider_identity: str = None
+    decider_identity: t.Union[str, None] = None
     """Identity of decider which acquired task."""
 
     @classmethod
@@ -894,7 +894,7 @@ class MarkerRecordedEvent(Event):
     decision_event_id: int
     """Marker record decision event ID."""
 
-    details: str = None
+    details: t.Union[str, None] = None
     """Attached marker data."""
 
     @classmethod
@@ -924,13 +924,13 @@ class LambdaFunctionScheduledEvent(Event):
     decision_event_id: int
     """Task schedule event ID."""
 
-    task_input: str = None
+    task_input: t.Union[str, None] = None
     """Lambda function input."""
 
     task_timeout: datetime.timedelta = _common.unset
     """Lambda function invocation timeout."""
 
-    control: str = None
+    control: t.Union[str, None] = None
     """Message from task scheduling decider."""
 
     @classmethod
@@ -981,7 +981,7 @@ class LambdaFunctionCompletedEvent(Event):
     task_started_event_id: int
     """Task start event ID."""
 
-    task_result: str = None
+    task_result: t.Union[str, None] = None
     """Lambda function invocation result."""
 
     @classmethod
@@ -1008,10 +1008,10 @@ class LambdaFunctionFailedEvent(Event):
     task_started_event_id: int
     """Task start event ID."""
 
-    reason: str = None
+    reason: t.Union[str, None] = None
     """Failure reason, usually for classification."""
 
-    details: str = None
+    details: t.Union[str, None] = None
     """Failure details, usually for explanation."""
 
     @classmethod
@@ -1134,7 +1134,7 @@ class RequestCancelExternalWorkflowExecutionFailedEvent(Event):
     decision_event_id: int
     """Cancellation request decision event ID."""
 
-    control: str = None
+    control: t.Union[str, None] = None
     """Message from cancellation requesting decider."""
 
     @classmethod
@@ -1165,7 +1165,7 @@ class RequestCancelExternalWorkflowExecutionInitiatedEvent(Event):
     decision_event_id: int
     """Cancellation request decision event ID."""
 
-    control: str = None
+    control: t.Union[str, None] = None
     """Message from cancellation requesting decider."""
 
     @classmethod
@@ -1264,7 +1264,7 @@ class SignalExternalWorkflowExecutionFailedEvent(Event):
     decision_event_id: int
     """External workflow signal decision event ID."""
 
-    control: str = None
+    control: t.Union[str, None] = None
     """Message from signalling decider."""
 
     @classmethod
@@ -1298,10 +1298,10 @@ class SignalExternalWorkflowExecutionInitiatedEvent(Event):
     decision_event_id: int
     """External workflow signal decision event ID."""
 
-    signal_input: str = None
+    signal_input: t.Union[str, None] = None
     """Attached signal data."""
 
-    control: str = None
+    control: t.Union[str, None] = None
     """Message from signalling decider."""
 
     @classmethod
@@ -1324,13 +1324,13 @@ class StartActivityTaskFailedEvent(Event):
 
     type: t.ClassVar[str] = "StartActivityTaskFailed"
 
-    task_id: str = None
+    task_id: t.Union[str, None] = None
     """ID of task to be started."""
 
-    cause: str = None
+    cause: t.Union[str, None] = None
     """Failure cause."""
 
-    task_scheduled_event_id: int = None
+    task_scheduled_event_id: t.Union[int, None] = None
     """Task schedule decision event ID."""
 
     @classmethod
@@ -1352,13 +1352,13 @@ class StartLambdaFunctionFailedEvent(Event):
 
     type: t.ClassVar[str] = "StartLambdaFunctionFailed"
 
-    cause: StartLambdaFailureCause = None
+    cause: t.Union[StartLambdaFailureCause, None] = None
     """Failure cause."""
 
-    message: str = None
+    message: t.Union[str, None] = None
     """Failure explanation."""
 
-    task_scheduled_event_id: int = None
+    task_scheduled_event_id: t.Union[int, None] = None
     """Task schedule decision event ID."""
 
     @classmethod
@@ -1396,7 +1396,7 @@ class StartChildWorkflowExecutionFailedEvent(Event):
     decision_event_id: int
     """Child workflow start decision event ID."""
 
-    control: str = None
+    control: t.Union[str, None] = None
     """Message from child execution starting decider."""
 
     @classmethod
@@ -1439,13 +1439,13 @@ class StartChildWorkflowExecutionInitiatedEvent(Event):
     decision_event_id: int
     """Child execution start decision event ID."""
 
-    execution_input: str = None
+    execution_input: t.Union[str, None] = None
     """Child execution input."""
 
-    execution_tags: t.List[str] = None
+    execution_tags: t.Union[t.List[str], None] = None
     """Child execution tags."""
 
-    control: str = None
+    control: t.Union[str, None] = None
     """Message from child execution starting decider."""
 
     @classmethod
@@ -1560,7 +1560,7 @@ class TimerStartedEvent(Event):
     decision_event_id: int
     """Timer start decision event ID."""
 
-    control: str = None
+    control: t.Union[str, None] = None
     """Message from timer starting decider."""
 
     @classmethod
@@ -1584,13 +1584,13 @@ class WorkflowExecutionCancelRequestedEvent(Event):
 
     type: t.ClassVar[str] = "WorkflowExecutionCancelRequested"
 
-    cause: ExecutionTerminationCause = None
+    cause: t.Union[ExecutionTerminationCause, None] = None
     """Cancellation request cause."""
 
-    cancelling_execution: "_executions.ExecutionId" = None
+    cancelling_execution: t.Union["_executions.ExecutionId", None] = None
     """Execution which requested the cancellation."""
 
-    cancel_decision_event_id: int = None
+    cancel_decision_event_id: t.Union[int, None] = None
     """Cancellation request decision event ID in execution which requested the
     cancellation.
     """
@@ -1622,7 +1622,7 @@ class WorkflowExecutionCancelledEvent(Event):
     decision_event_id: int
     """Execution cancel decision event ID."""
 
-    details: str = None
+    details: t.Union[str, None] = None
     """Execution cancellation details, usually for explanation."""
 
     @classmethod
@@ -1645,7 +1645,7 @@ class WorkflowExecutionCompletedEvent(Event):
     decision_event_id: int
     """Execution complete decision event ID."""
 
-    execution_result: str = None
+    execution_result: t.Union[str, None] = None
     """Execution result."""
 
     @classmethod
@@ -1679,10 +1679,10 @@ class WorkflowExecutionContinuedAsNewEvent(Event):
     decision_event_id: int
     """Continue as new execution decision event ID."""
 
-    execution_input: str = None
+    execution_input: t.Union[str, None] = None
     """New execution input."""
 
-    execution_tags: t.List[str] = None
+    execution_tags: t.Union[t.List[str], None] = None
     """New execution tags."""
 
     @classmethod
@@ -1713,10 +1713,10 @@ class WorkflowExecutionFailedEvent(Event):
     decision_event_id: int
     """Execution fail decision event ID."""
 
-    reason: str = None
+    reason: t.Union[str, None] = None
     """Failure reason, usually for classification."""
 
-    details: str = None
+    details: t.Union[str, None] = None
     """Failure details, usually for explanation."""
 
     @classmethod
@@ -1740,13 +1740,13 @@ class WorkflowExecutionSignaledEvent(Event):
     signal_name: str
     """Signal name."""
 
-    signal_input: str = None
+    signal_input: t.Union[str, None] = None
     """Attached signal data."""
 
-    signalling_execution: "_executions.ExecutionId" = None
+    signalling_execution: t.Union["_executions.ExecutionId", None] = None
     """Execution which sent the signal."""
 
-    signal_decision_event_id: int = None
+    signal_decision_event_id: t.Union[int, None] = None
     """Signal decision event ID in execution which sent the signal."""
 
     @classmethod
@@ -1779,19 +1779,19 @@ class WorkflowExecutionStartedEvent(Event):
     guaranteed).
     """
 
-    execution_input: str = None
+    execution_input: t.Union[str, None] = None
     """Execution input."""
 
-    execution_tags: t.List[str] = None
+    execution_tags: t.Union[t.List[str], None] = None
     """Execution tags."""
 
-    continued_execution_run_id: str = None
+    continued_execution_run_id: t.Union[str, None] = None
     """Run ID of execution which this execution continues from."""
 
-    parent_execution: "_executions.ExecutionId" = None
+    parent_execution: t.Union["_executions.ExecutionId", None] = None
     """Parent execution (which started this execution)."""
 
-    parent_initiated_event_id: int = None
+    parent_initiated_event_id: t.Union[int, None] = None
     """ID of event in parent execution which begins the starting of this
     execution."""
 
@@ -1827,13 +1827,13 @@ class WorkflowExecutionTerminatedEvent(Event):
     child_execution_policy: "_executions.ChildExecutionTerminationPolicy"
     """Child execution policy (how open child executions were handled)."""
 
-    cause: ExecutionTerminationCause = None
+    cause: t.Union[ExecutionTerminationCause, None] = None
     """Failure cause."""
 
-    reason: str = None
+    reason: t.Union[str, None] = None
     """Termination reason, usually for classification."""
 
-    details: str = None
+    details: t.Union[str, None] = None
     """Termination details, usually for explanation."""
 
     @classmethod
@@ -1898,7 +1898,7 @@ def get_execution_history(
     execution: "_executions.ExecutionId",
     domain: str,
     reverse: bool = False,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> t.Generator[Event, None, None]:
     """Get workflow execution history; retrieved semi-lazily.
 
@@ -1925,7 +1925,7 @@ def get_execution_history(
 def get_last_execution_history_event(
     execution: "_executions.ExecutionId",
     domain: str,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> Event:
     """Get last workflow execution history event.
 
diff --git a/src/swf_typed/_state.py b/src/swf_typed/_state.py
index 252a09a..54c1a96 100644
--- a/src/swf_typed/_state.py
+++ b/src/swf_typed/_state.py
@@ -78,34 +78,34 @@ class TaskState:
     scheduled: datetime.datetime
     """Task scheduled date."""
 
-    started: datetime.datetime = None
+    started: t.Union[datetime.datetime, None] = None
     """Task start date."""
 
-    ended: datetime.datetime = None
+    ended: t.Union[datetime.datetime, None] = None
     """Task end date."""
 
-    input: str = None
+    input: t.Union[str, None] = None
     """Task input."""
 
-    worker_identity: str = None
+    worker_identity: t.Union[str, None] = None
     """Identity of worker which acquired task."""
 
     cancel_requested: bool = False
     """Task cancellation has been requested."""
 
-    result: str = None
+    result: t.Union[str, None] = None
     """Task result."""
 
-    timeout_type: "_history.TimeoutType" = None
+    timeout_type: t.Union["_history.TimeoutType", None] = None
     """Task timeout type."""
 
-    failure_reason: str = None
+    failure_reason: t.Union[str, None] = None
     """Task failure reason."""
 
-    stop_details: str = None
+    stop_details: t.Union[str, None] = None
     """Task ended details."""
 
-    decider_control: str = None
+    decider_control: t.Union[str, None] = None
     """Message from decider attached to task."""
 
     @property
@@ -130,28 +130,28 @@ class LambdaTaskState:
     scheduled: datetime.datetime
     """Task schedule date."""
 
-    started: datetime.datetime = None
+    started: t.Union[datetime.datetime, None] = None
     """Lambda function invocation date."""
 
-    ended: datetime.datetime = None
+    ended: t.Union[datetime.datetime, None] = None
     """Lambda function invocation end date."""
 
-    timeout: datetime.timedelta = None
+    timeout: t.Union[datetime.timedelta, None] = None
     """Lambda function invocation timeout."""
 
-    input: str = None
+    input: t.Union[str, None] = None
     """Lambda function input."""
 
-    result: str = None
+    result: t.Union[str, None] = None
     """Lambda function result."""
 
-    failure_reason: str = None
+    failure_reason: t.Union[str, None] = None
     """Lambda function error reason."""
 
-    stop_details: str = None
+    stop_details: t.Union[str, None] = None
     """Lambda function ended details."""
 
-    decider_control: str = None
+    decider_control: t.Union[str, None] = None
     """Message from decider attached to task."""
 
     @property
@@ -179,25 +179,25 @@ class ChildExecutionState:
     started: datetime.datetime
     """Child execution start date."""
 
-    ended: datetime.datetime = None
+    ended: t.Union[datetime.datetime, None] = None
     """Child execution end date."""
 
-    input: str = None
+    input: t.Union[str, None] = None
     """Child execution input."""
 
-    result: str = None
+    result: t.Union[str, None] = None
     """Child execution result."""
 
-    timeout_type: "_history.TimeoutType" = None
+    timeout_type: t.Union["_history.TimeoutType", None] = None
     """Child execution timeout type."""
 
-    failure_reason: str = None
+    failure_reason: t.Union[str, None] = None
     """Child execution failure reason."""
 
-    stop_details: str = None
+    stop_details: t.Union[str, None] = None
     """Child execution ended details."""
 
-    decider_control: str = None
+    decider_control: t.Union[str, None] = None
     """Message from decider attached to child execution."""
 
 
@@ -217,13 +217,13 @@ class TimerState:
     started: datetime.datetime
     """Timer start date."""
 
-    ended: datetime.datetime = None
+    ended: t.Union[datetime.datetime, None] = None
     """Timer finish date."""
 
-    input: str = None
+    input: t.Union[str, None] = None
     """Timer input."""
 
-    decider_control: str = None
+    decider_control: t.Union[str, None] = None
     """Message from decider attached to timer."""
 
 
@@ -237,7 +237,7 @@ class SignalState:
     received: datetime.datetime
     """Signal date."""
 
-    input: str = None
+    input: t.Union[str, None] = None
     """Signal input."""
 
     is_new: bool = True
@@ -254,7 +254,7 @@ class MarkerState:
     recorded: datetime.datetime
     """Marker record date."""
 
-    details: str = None
+    details: t.Union[str, None] = None
     """Marker details."""
 
     is_new: bool = True
@@ -277,7 +277,7 @@ class ExecutionState:
     started: datetime.datetime
     """Execution start date."""
 
-    ended: datetime.datetime = None
+    ended: t.Union[datetime.datetime, None] = None
     """Execution end date."""
 
     tasks: t.List[t.Union[TaskState, LambdaTaskState]] = dataclasses.field(
@@ -302,22 +302,22 @@ class ExecutionState:
     decision_failures: t.List[DecisionFailure] = dataclasses.field(default_factory=list)
     """Execution decision failures."""
 
-    input: str = None
+    input: t.Union[str, None] = None
     """Execution input."""
 
     cancel_requested: bool = False
     """Execution cancellation has been requested."""
 
-    result: str = None
+    result: t.Union[str, None] = None
     """Execution result."""
 
-    failure_reason: str = None
+    failure_reason: t.Union[str, None] = None
     """Execution failure reason."""
 
-    stop_details: str = None
+    stop_details: t.Union[str, None] = None
     """Execution ended details."""
 
-    continuing_execution_run_id: str = None
+    continuing_execution_run_id: t.Union[str, None] = None
     """ID of execution continuing this execution."""
 
 
diff --git a/src/swf_typed/_tasks.py b/src/swf_typed/_tasks.py
index 10480a4..3849c0a 100644
--- a/src/swf_typed/_tasks.py
+++ b/src/swf_typed/_tasks.py
@@ -125,7 +125,7 @@ class WorkerTask(_common.Deserialisable):
     task_started_execution_history_event_id: int
     """History event ID for task start."""
 
-    input: str = None
+    input: t.Union[str, None] = None
     """Task input."""
 
     @classmethod
@@ -146,7 +146,7 @@ def from_api(cls, data) -> "WorkerTask":
 def get_number_of_pending_tasks(
     task_list: str,
     domain: str,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> int:
     """Get the number of pending activity tasks.
 
@@ -173,9 +173,9 @@ def get_number_of_pending_tasks(
 def request_task(
     task_list: str,
     domain: str,
-    worker_identity: str = None,
-    no_tasks_callback: t.Callable[[], None] = None,
-    client: "botocore.client.BaseClient" = None,
+    worker_identity: t.Union[str, None] = None,
+    no_tasks_callback: t.Union[t.Callable[[], None], None] = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> WorkerTask:
     """Request (poll for) an activity task; blocks until task is received.
 
@@ -208,8 +208,8 @@ def request_task(
 
 def send_heartbeat(
     token: str,
-    details: str = None,
-    client: "botocore.client.BaseClient" = None,
+    details: t.Union[str, None] = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> None:
     """Send a heartbeat to SWF for the current activity task.
 
@@ -233,8 +233,8 @@ def send_heartbeat(
 
 def cancel_task(
     token: str,
-    details: str = None,
-    client: "botocore.client.BaseClient" = None,
+    details: t.Union[str, None] = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> None:
     """Cancel the current activity task.
 
@@ -255,8 +255,8 @@ def cancel_task(
 
 def complete_task(
     token: str,
-    result: str = None,
-    client: "botocore.client.BaseClient" = None,
+    result: t.Union[str, None] = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> None:
     """Complete the current activity task.
 
@@ -277,9 +277,9 @@ def complete_task(
 
 def fail_task(
     token: str,
-    reason: str = None,
-    details: str = None,
-    client: "botocore.client.BaseClient" = None,
+    reason: t.Union[str, None] = None,
+    details: t.Union[str, None] = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> None:
     """Fail the current activity task.
 
diff --git a/src/swf_typed/_workflows.py b/src/swf_typed/_workflows.py
index edef6c1..9ec1734 100644
--- a/src/swf_typed/_workflows.py
+++ b/src/swf_typed/_workflows.py
@@ -43,10 +43,10 @@ class WorkflowInfo(_common.Deserialisable):
     created: datetime.datetime
     """Creation date."""
 
-    description: str = None
+    description: t.Union[str, None] = None
     """Workflow description."""
 
-    deprecated: datetime.datetime = None
+    deprecated: t.Union[datetime.datetime, None] = None
     """Deprecation date."""
 
     @classmethod
@@ -109,7 +109,7 @@ def get_api_args(self):
 def delete_workflow(
     workflow: WorkflowId,
     domain: str,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> None:
     """Delete a (deprecated/inactive) workflow type.
 
@@ -126,7 +126,7 @@ def delete_workflow(
 def deprecate_workflow(
     workflow: WorkflowId,
     domain: str,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> None:
     """Deprecate (deactivate) a workflow type.
 
@@ -143,7 +143,7 @@ def deprecate_workflow(
 def describe_workflow(
     workflow: WorkflowId,
     domain: str,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> WorkflowDetails:
     """Describe a workflow type.
 
@@ -166,9 +166,9 @@ def describe_workflow(
 def list_workflows(
     domain: str,
     deprecated: bool = False,
-    workflow_filter: WorkflowIdFilter = None,
+    workflow_filter: t.Union[WorkflowIdFilter, None] = None,
     reverse: bool = False,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> t.Generator[WorkflowInfo, None, None]:
     """List workflow types; retrieved semi-lazily.
 
@@ -201,9 +201,9 @@ def list_workflows(
 def register_workflow(
     workflow: WorkflowId,
     domain: str,
-    description: str = None,
-    default_execution_configuration: DefaultExecutionConfiguration = None,
-    client: "botocore.client.BaseClient" = None,
+    description: t.Union[str, None] = None,
+    default_execution_configuration: t.Union[DefaultExecutionConfiguration, None] = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> None:
     """Register a new workflow type.
 
@@ -234,7 +234,7 @@ def register_workflow(
 def undeprecate_workflow(
     workflow: WorkflowId,
     domain: str,
-    client: "botocore.client.BaseClient" = None,
+    client: t.Union["botocore.client.BaseClient", None] = None,
 ) -> None:
     """Undeprecate (reactivate) a workflow type.