diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index b9b2e25d1..653ab0dbf 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -114,9 +114,9 @@ class FederationHandler(BaseHandler):
 
         event_ids = set()
         if state:
-            event_ids += {e.event_id for e in state}
+            event_ids |= {e.event_id for e in state}
         if auth_chain:
-            event_ids += {e.event_id for e in auth_chain}
+            event_ids |= {e.event_id for e in auth_chain}
 
         seen_ids = (yield self.store.have_events(event_ids)).keys()