diff --git a/deepmd/dpmodel/output_def.py b/deepmd/dpmodel/output_def.py
index 5e1255524c..1359d532ec 100644
--- a/deepmd/dpmodel/output_def.py
+++ b/deepmd/dpmodel/output_def.py
@@ -197,7 +197,7 @@ def __init__(
         r_hessian: bool = False,
         magnetic: bool = False,
         intensive: bool = False,
-        sub_var_name: list[str] = None,
+        sub_var_name: list[str] = [],
     ) -> None:
         self.name = name
         self.shape = list(shape)