diff --git a/synapse/storage/data_stores/main/stream.py b/synapse/storage/data_stores/main/stream.py
index 223ce7fedbda..9fa5e1f203b1 100644
--- a/synapse/storage/data_stores/main/stream.py
+++ b/synapse/storage/data_stores/main/stream.py
@@ -556,7 +556,7 @@ def get_room_event_before_stream_ordering(self, room_id, stream_ordering):
         return self.db.runInteraction(
             "get_room_event_before_stream_ordering",
             self.get_room_event_around_stream_ordering_txn,
-            room_id, stream_ordering, "f",
+            room_id, stream_ordering, "b",
         )
 
     def get_room_event_around_stream_ordering_txn(