diff --git a/synapse/storage/deviceinbox.py b/synapse/storage/deviceinbox.py
index b729b7106ecb..f640e7371460 100644
--- a/synapse/storage/deviceinbox.py
+++ b/synapse/storage/deviceinbox.py
@@ -128,6 +128,8 @@ def add_messages_txn(txn, now_ms, stream_id):
                     user_id, stream_id
                 )
 
+        defer.returnValue(stream_id)
+
     def _add_messages_to_local_device_inbox_txn(self, txn, stream_id,
                                                 messages_by_user_then_device):
         sql = (