diff --git a/synapse/handlers/federation_event.py b/synapse/handlers/federation_event.py
index 799aaf2e1a12..9ec90ac8c101 100644
--- a/synapse/handlers/federation_event.py
+++ b/synapse/handlers/federation_event.py
@@ -1700,10 +1700,13 @@ async def _run_push_actions_and_persist_event(
             context: The event context.
             backfilled: True if the event was backfilled.
         """
+        # this method should not be called on outliers (those code paths call
+        # persist_events_and_notify directly.)
+        assert not event.internal_metadata.outlier
+
         try:
             if (
-                not event.internal_metadata.is_outlier()
-                and not backfilled
+                not backfilled
                 and not context.rejected
                 and (await self._store.get_min_depth(event.room_id)) <= event.depth
             ):