diff --git a/shopfloor/actions/location_content_transfer_sorter.py b/shopfloor/actions/location_content_transfer_sorter.py
index 1ad4aa1f8b1..683bc737cb8 100644
--- a/shopfloor/actions/location_content_transfer_sorter.py
+++ b/shopfloor/actions/location_content_transfer_sorter.py
@@ -56,8 +56,7 @@ def package_levels(self):
             )
         )
 
-    @staticmethod
-    def _sort_key(content):
+    def _sort_key(self, content):
         # content can be either a move line, either a package
         # level
         return (