diff --git a/src/llmtuner/chat/vllm_engine.py b/src/llmtuner/chat/vllm_engine.py
index faf8c9fe..aaaad2f1 100644
--- a/src/llmtuner/chat/vllm_engine.py
+++ b/src/llmtuner/chat/vllm_engine.py
@@ -100,8 +100,9 @@ class VllmEngine(BaseEngine):
         max_new_tokens = input_kwargs.pop("max_new_tokens", None)
         stop = input_kwargs.pop("stop", None)
 
+        max_tokens = self.generating_args["max_new_tokens"] or self.generating_args["max_length"]
         if max_length:
-            max_tokens = max_length - prompt_length
+            max_tokens = max_length - prompt_length if max_length > prompt_length else 1
 
         if max_new_tokens:
             max_tokens = max_new_tokens