diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 06d453017..ed0fa51e7 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -1096,7 +1096,7 @@ class FederationHandler(BaseHandler):
                     if prev_id != event.event_id:
                         results[(event.type, event.state_key)] = prev_id
                 else:
-                    results.pop((event.type, event.state_key))
+                    results.pop((event.type, event.state_key), None)
 
             defer.returnValue(results.values())
         else: