diff --git a/synapse/storage/event_federation.py b/synapse/storage/event_federation.py
index 2880850506..2f913adf20 100644
--- a/synapse/storage/event_federation.py
+++ b/synapse/storage/event_federation.py
@@ -378,7 +378,7 @@ class EventFederationStore(SQLBaseStore):
             room_id, repr(event_list), limit
         )
 
-        event_results = event_list
+        event_results = set(event_list)
 
         front = event_list