diff --git a/applications/ColossalChat/rl_example.py b/applications/ColossalChat/rl_example.py
index d7b7c2a5d..ffec153b2 100644
--- a/applications/ColossalChat/rl_example.py
+++ b/applications/ColossalChat/rl_example.py
@@ -319,6 +319,7 @@ if __name__ == "__main__":
                 else None
             ),
         }
+        assert args.n_behind == 0, "DAPO does not support N behined."
     else:
         raise ValueError(f"Unsupported algorithm: {args.algo}")