diff --git a/llms/mlx_lm/tuner/datasets.py b/llms/mlx_lm/tuner/datasets.py
index 0f23c2a4..98f01ef3 100644
--- a/llms/mlx_lm/tuner/datasets.py
+++ b/llms/mlx_lm/tuner/datasets.py
@@ -19,7 +19,7 @@ class DPODataset:
         prompt_key: str = "prompt",
         chosen_key: str = "chosen",
         rejected_key: str = "rejected",
-        system_key: str = None
+        system_key: str = "system",
     ):
 
         self._chosen_data = []