diff --git a/src/marshmallow/fields.py b/src/marshmallow/fields.py
index 5611b24a4..52d13dcf7 100644
--- a/src/marshmallow/fields.py
+++ b/src/marshmallow/fields.py
@@ -387,7 +387,9 @@ def _bind_to_schema(self, field_name, schema):
             self.parent.root if isinstance(self.parent, FieldABC) else self.parent
         )
 
-    def _serialize(self, value: typing.Any, attr: str, obj: typing.Any, **kwargs):
+    def _serialize(
+        self, value: typing.Any, attr: str | None, obj: typing.Any, **kwargs
+    ):
         """Serializes ``value`` to a basic Python datatype. Noop by default.
         Concrete :class:`Field` classes should implement this method.