diff --git a/python/packages/autogen-core/src/autogen_core/components/tools/_function_tool.py b/python/packages/autogen-core/src/autogen_core/components/tools/_function_tool.py
index 131cea7483..e537deeba9 100644
--- a/python/packages/autogen-core/src/autogen_core/components/tools/_function_tool.py
+++ b/python/packages/autogen-core/src/autogen_core/components/tools/_function_tool.py
@@ -46,5 +46,6 @@ class FunctionTool(BaseTool[BaseModel, BaseModel]):
                 cancellation_token.link_future(future)
                 result = await future
 
-        assert isinstance(result, self.return_type())
+        if not isinstance(result, self.return_type()):
+            raise ValueError(f"Expected return type {self.return_type()}, got {type(result)}")
         return result