diff --git a/fimfarchive/selectors.py b/fimfarchive/selectors.py
index de78f0c..a579337 100644
--- a/fimfarchive/selectors.py
+++ b/fimfarchive/selectors.py
@@ -38,7 +38,7 @@ class Selector:
     Picks one of the two supplied stories.
     """
 
-    def __call__(old, new):
+    def __call__(self, old, new):
         """
         Returns either the old or the new story.