diff --git a/src/dbus_fast/service.py b/src/dbus_fast/service.py
index 7f5b474..0eff2b7 100644
--- a/src/dbus_fast/service.py
+++ b/src/dbus_fast/service.py
@@ -504,7 +504,9 @@ class ServiceInterface:
 
     @staticmethod
     def _fn_result_to_body(
-        result: List[Any], signature_tree: SignatureTree, replace_fds: bool = True
+        result: Optional[List[Any]],
+        signature_tree: SignatureTree,
+        replace_fds: bool = True,
     ) -> Tuple[List[Any], List[int]]:
         """The high level interfaces may return single values which may be
         wrapped in a list to be a message body. Also they may return fds