1 files changed, 9 insertions, 7 deletions
diff --git a/synapse/storage/databases/main/events_worker.py b/synapse/storage/databases/main/events_worker.py
index 5687448e3d..8c63a0dc4d 100644
--- a/synapse/storage/databases/main/events_worker.py
+++ b/synapse/storage/databases/main/events_worker.py
@@ -847,13 +847,15 @@ class EventsWorkerStore(SQLBaseStore):
         """Given a list of event ids, check if we have already processed and
         stored them as non outliers.
         """
-        rows = yield self.db_pool.simple_select_many_batch(
-            table="events",
-            retcols=("event_id",),
-            column="event_id",
-            iterable=list(event_ids),
-            keyvalues={"outlier": False},
-            desc="have_events_in_timeline",
+        rows = yield defer.ensureDeferred(
+            self.db_pool.simple_select_many_batch(
+                table="events",
+                retcols=("event_id",),
+                column="event_id",
+                iterable=list(event_ids),
+                keyvalues={"outlier": False},
+                desc="have_events_in_timeline",
+            )
         )
 
         return {r["event_id"] for r in rows}