diff --git a/src/agents/extensions/handoff_prompt.py b/src/agents/extensions/handoff_prompt.py
index cfb5ca7ec..2f3b3ef00 100644
--- a/src/agents/extensions/handoff_prompt.py
+++ b/src/agents/extensions/handoff_prompt.py
@@ -16,4 +16,7 @@ def prompt_with_handoff_instructions(prompt: str) -> str:
     """
     Add recommended instructions to the prompt for agents that use handoffs.
     """
+    if not isinstance(prompt, str):
+        raise TypeError(f"prompt must be a string, got {type(prompt).__name__}")
+
     return f"{RECOMMENDED_PROMPT_PREFIX}\n\n{prompt}"