1 files changed, 16 insertions, 4 deletions
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 986ffed3d5..44e70c6c3c 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -781,15 +781,27 @@ class FederationHandler:
 
         # Send the signed event back to the room, and potentially receive some
         # further information about the room in the form of partial state events
-        stripped_room_state = await self.federation_client.send_knock(
-            target_hosts, event
-        )
+        knock_response = await self.federation_client.send_knock(target_hosts, event)
 
         # Store any stripped room state events in the "unsigned" key of the event.
         # This is a bit of a hack and is cribbing off of invites. Basically we
         # store the room state here and retrieve it again when this event appears
         # in the invitee's sync stream. It is stripped out for all other local users.
-        event.unsigned["knock_room_state"] = stripped_room_state["knock_state_events"]
+        stripped_room_state = (
+            knock_response.get("knock_room_state")
+            # Since v1.37, Synapse incorrectly used "knock_state_events" for this field.
+            # Thus, we also check for a 'knock_state_events' to support old instances.
+            # See https://github.com/matrix-org/synapse/issues/14088.
+            or knock_response.get("knock_state_events")
+        )
+
+        if stripped_room_state is None:
+            raise KeyError(
+                "Missing 'knock_room_state' (or legacy 'knock_state_events') field in "
+                "send_knock response"
+            )
+
+        event.unsigned["knock_room_state"] = stripped_room_state
 
         context = EventContext.for_outlier(self._storage_controllers)
         stream_id = await self._federation_event_handler.persist_events_and_notify(