diff --git a/masque/library.py b/masque/library.py
index 70d6772..e11ef5b 100644
--- a/masque/library.py
+++ b/masque/library.py
@@ -434,7 +434,7 @@ class MutableLibrary(Library, MutableMapping[str, 'Pattern'], metaclass=ABCMeta)
 
     def name_and_set(
             self,
-            base_name: str
+            base_name: str,
             value: Union['Pattern', Callable[[], 'Pattern']],
             ) -> str:
         """