diff --git a/synapse/storage/receipts.py b/synapse/storage/receipts.py
index c2a6613a62..40530632c6 100644
--- a/synapse/storage/receipts.py
+++ b/synapse/storage/receipts.py
@@ -52,7 +52,7 @@ class ReceiptsWorkerStore(SQLBaseStore):
         Returns:
             int
         """
-        pass
+        raise NotImplementedError()
 
     @cachedInlineCallbacks()
     def get_users_with_read_receipts_in_room(self, room_id):