diff --git a/synapse/storage/event_federation.py b/synapse/storage/event_federation.py
index 43b5b4998..519059c30 100644
--- a/synapse/storage/event_federation.py
+++ b/synapse/storage/event_federation.py
@@ -152,7 +152,7 @@ class EventFederationStore(SQLBaseStore):
         txn.execute(sql, (room_id, ))
 
         results = []
-        for event_id, depth in txn:
+        for event_id, depth in txn.fetchall():
             hashes = self._get_event_reference_hashes_txn(txn, event_id)
             prev_hashes = {
                 k: encode_base64(v) for k, v in hashes.items()