diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 8f444da871..1d07e4d02b 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -252,6 +252,8 @@ class FederationHandler(BaseHandler):
             except:
                 return False
 
+        # Parses mapping `event_id -> (type, state_key) -> state event_id`
+        # to get all state ids that we're interested in.
         event_map = yield self.store.get_events([
             e_id
             for key_to_eid in event_to_state_ids.values()