1 files changed, 1 insertions, 1 deletions
diff --git a/synapse/storage/databases/main/events.py b/synapse/storage/databases/main/events.py
index 0c1ed75240..bc8474a589 100644
--- a/synapse/storage/databases/main/events.py
+++ b/synapse/storage/databases/main/events.py
@@ -222,7 +222,7 @@ class PersistEventsStore:
 
             for room_id, latest_event_ids in new_forward_extremities.items():
                 self.store.get_latest_event_ids_in_room.prefill(
-                    (room_id,), list(latest_event_ids)
+                    (room_id,), frozenset(latest_event_ids)
                 )
 
     async def _get_events_which_are_prevs(self, event_ids: Iterable[str]) -> List[str]: