diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index a72c92c62..39d2bee8d 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -1102,6 +1102,8 @@ class FederationHandler(BaseHandler):
             user_id,
             "leave"
         )
+        # Mark as outlier as we don't have any state for this event; we're not
+        # even in the room.
         event.internal_metadata.outlier = True
         event = self._sign_event(event)