diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 2338c3a72b..fa7ea63858 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -101,6 +101,9 @@ class FederationHandler(BaseHandler):
     def on_receive_pdu(self, origin, pdu, state=None, auth_chain=None):
         """ Called by the ReplicationLayer when we have a new pdu. We need to
         do auth checks and put it through the StateHandler.
+
+        auth_chain and state are None if we already have the necessary state
+        and prev_events in the db
         """
         event = pdu