diff --git a/vllm/entrypoints/openai/completion/protocol.py b/vllm/entrypoints/openai/completion/protocol.py
index 30a4f20084..1d61ca3c59 100644
--- a/vllm/entrypoints/openai/completion/protocol.py
+++ b/vllm/entrypoints/openai/completion/protocol.py
@@ -346,6 +346,14 @@ class CompletionRequest(OpenAIBaseModel):
             thinking_token_budget=self.thinking_token_budget,
         )
 
+    @model_validator(mode="before")
+    @classmethod
+    def normalize_null_max_tokens(cls, data):
+        if isinstance(data, dict) and data.get("max_tokens") is None:
+            data = data.copy()
+            data["max_tokens"] = cls.model_fields["max_tokens"].default
+        return data
+
     @model_validator(mode="before")
     @classmethod
     def validate_response_format(cls, data):