diff --git a/src/rfdetr/config.py b/src/rfdetr/config.py
index 8ba2e8ee..c45114bf 100644
--- a/src/rfdetr/config.py
+++ b/src/rfdetr/config.py
@@ -412,7 +412,7 @@ class TrainConfig(BaseModel):
     clearml: bool = False  # Not yet implemented — reserved for future use.
     project: Optional[str] = None
     run: Optional[str] = None
-    class_names: List[str] = None
+    class_names: Optional[List[str]] = None
     run_test: bool = False
     segmentation_head: bool = False
     eval_max_dets: int = 500