diff --git a/synapse/federation/send_queue.py b/synapse/federation/send_queue.py
index 3dcc629d44a2..9f1142b5a9d7 100644
--- a/synapse/federation/send_queue.py
+++ b/synapse/federation/send_queue.py
@@ -75,7 +75,7 @@ def __init__(self, hs):
         # changes. ARGH.
         def register(name, queue):
             LaterGauge("synapse_federation_send_queue_%s_size" % (queue_name,),
-                       "", lambda: len(queue))
+                       "", [], lambda: len(queue))
 
         for queue_name in [
             "presence_map", "presence_changed", "keyed_edu", "keyed_edu_changed",