diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index 97239b1b491..41b00a5cf7b 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -1744,7 +1744,7 @@ async def get_new_events(
 
             # We know stream_ordering must be not None here, as its been
             # persisted, but mypy doesn't know that
-            events.sort(key=lambda e: e.internal_metadata.stream_ordering or 0)
+            events.sort(key=lambda e: cast(int, e.internal_metadata.stream_ordering))
 
             if limit:
                 events[:] = events[:limit]