1 files changed, 1 insertions, 0 deletions
diff --git a/synapse/handlers/device.py b/synapse/handlers/device.py
index 5c06073901..6f7963df43 100644
--- a/synapse/handlers/device.py
+++ b/synapse/handlers/device.py
@@ -859,6 +859,7 @@ class DeviceHandler(DeviceWorkerHandler):
         known_hosts_at_join = await self.store.get_partial_state_servers_at_join(
             room_id
         )
+        assert known_hosts_at_join is not None
         potentially_changed_hosts.difference_update(known_hosts_at_join)
 
         potentially_changed_hosts.discard(self.server_name)