diff --git a/fastchat/utils.py b/fastchat/utils.py
index d3531928f..13c9f1453 100644
--- a/fastchat/utils.py
+++ b/fastchat/utils.py
@@ -330,7 +330,7 @@ def parse_gradio_auth_creds(filename: str):
 
 def is_partial_stop(output: str, stop_str: str):
     """Check whether the output contains a partial stop str."""
-    for i in range(0, min(len(output), len(stop_str))):
+    for i in range(1, min(len(output), len(stop_str)) + 1):
         if stop_str.startswith(output[-i:]):
             return True
     return False