diff --git a/synapse/storage/user_directory.py b/synapse/storage/user_directory.py
index 5d402189e..745e6f26e 100644
--- a/synapse/storage/user_directory.py
+++ b/synapse/storage/user_directory.py
@@ -393,7 +393,7 @@ class UserDirectoryStore(BackgroundUpdateStore):
                         txn,
                         "users_in_public_rooms",
                         keyvalues={"user_id": user_id},
-                        values=None,
+                        values={},
                     )
 
             for user_id, other_user_id in user_id_tuples: