diff --git a/synapse/federation/sender/per_destination_queue.py b/synapse/federation/sender/per_destination_queue.py
index a9e77793d031..4d96f026c664 100644
--- a/synapse/federation/sender/per_destination_queue.py
+++ b/synapse/federation/sender/per_destination_queue.py
@@ -362,6 +362,8 @@ def _get_new_device_messages(self, limit):
             for content in results
         ]
 
+        assert len(edus) <= limit, "get_devices_by_remote returned too many EDUs"
+
         last_device_stream_id = self._last_device_stream_id
         to_device_stream_id = self._store.get_to_device_stream_token()
         contents, stream_id = yield self._store.get_new_device_msgs_for_remote(