diff --git a/synapse/visibility.py b/synapse/visibility.py
index 202daa6800..9b97ea2b83 100644
--- a/synapse/visibility.py
+++ b/synapse/visibility.py
@@ -248,6 +248,8 @@ def filter_events_for_server(store, server_name, events):
             )
             return prune_event(event)
 
+        # state will be None if we decided we didn't need to filter by
+        # room membership.
         if not state:
             return event