diff --git a/gc/mmtk/mmtk.c b/gc/mmtk/mmtk.c
index 38e730a..0321053 100644
--- a/gc/mmtk/mmtk.c
+++ b/gc/mmtk/mmtk.c
@@ -83,6 +83,12 @@ RB_THREAD_LOCAL_SPECIFIER VALUE marking_parent_object;
 # error We currently need language-supported TLS
 #endif
 
+#ifdef MMTK_DEBUG
+# define MMTK_ASSERT(expr, ...) RUBY_ASSERT_ALWAYS(expr, #expr RBIMPL_VA_OPT_ARGS(__VA_ARGS__))
+#else
+# define MMTK_ASSERT(expr, ...) ((void)0)
+#endif
+
 #include <pthread.h>
 
 static void
@@ -975,6 +981,9 @@ rb_gc_impl_writebarrier(void *objspace_ptr, VALUE a, VALUE b)
     }
 #endif
 
+    MMTK_ASSERT(BUILTIN_TYPE(a) != T_NONE);
+    MMTK_ASSERT(BUILTIN_TYPE(b) != T_NONE);
+
     mmtk_object_reference_write_post(cache->mutator, (MMTk_ObjectReference)a);
 }