diff --git a/synapse/federation/federation_client.py b/synapse/federation/federation_client.py
index 178e1e7e0a..4fd16228d6 100644
--- a/synapse/federation/federation_client.py
+++ b/synapse/federation/federation_client.py
@@ -1399,6 +1399,7 @@ class FederationClient(FederationBase):
                     "event": pdu.get_pdu_json(time_now),
                     "room_version": room_version.identifier,
                     "invite_room_state": [
+                        # Use full PDU's according to MSC4311
                         state_event.get_pdu_json(time_now)
                         for state_event in state_events.values()
                     ],