diff --git a/vllm/entrypoints/openai/protocol.py b/vllm/entrypoints/openai/protocol.py
index 7c2aa707775ff..18b0df6367ee6 100644
--- a/vllm/entrypoints/openai/protocol.py
+++ b/vllm/entrypoints/openai/protocol.py
@@ -55,7 +55,7 @@ class UsageInfo(BaseModel):
 
 class ChatCompletionRequest(BaseModel):
     model: str
-    messages: Union[str, List[Dict[str, str]]]
+    messages: List[Dict[str, str]]
     temperature: Optional[float] = 0.7
     top_p: Optional[float] = 1.0
     n: Optional[int] = 1