diff --git a/nautilus_trader/persistence/catalog/parquet.py b/nautilus_trader/persistence/catalog/parquet.py
index bd5567ce9182..d9793df276eb 100644
--- a/nautilus_trader/persistence/catalog/parquet.py
+++ b/nautilus_trader/persistence/catalog/parquet.py
@@ -337,7 +337,7 @@ def key(obj: Any) -> tuple[str, str | None]:
                 return name, obj.instrument_id.value
             return name, None
 
-        def obj_to_type(obj) -> type:
+        def obj_to_type(obj: Data) -> type:
             return type(obj) if not isinstance(obj, CustomData) else obj.data.__class__
 
         name_to_cls = {cls.__name__: cls for cls in {obj_to_type(d) for d in data}}