diff --git a/openai_chat.py b/openai_chat.py
index 8305b0f..d4fd3d9 100644
--- a/openai_chat.py
+++ b/openai_chat.py
@@ -62,7 +62,7 @@ class OpenaiChat(PyOperator):
                     elif k == 'system':
                         new_m = {'role': 'system', 'content': v}
                     else:
-                        'Invalid message key: only accept key value from ["system", "question", "answer"].'
+                        raise KeyError('Invalid message key: only accept key value from ["system", "question", "answer"].')
                     new_messages.append(new_m)
         return new_messages