diff --git a/changelog.d/12024.bugfix b/changelog.d/12024.bugfix
new file mode 100644
index 000000000000..59bcdb93a57f
--- /dev/null
+++ b/changelog.d/12024.bugfix
@@ -0,0 +1 @@
+Fix 500 error with Postgres when looking backwards with the [MSC3030](https://github.com/matrix-org/matrix-doc/pull/3030) `/timestamp_to_event?dir=b` endpoint.
diff --git a/synapse/storage/databases/main/events_worker.py b/synapse/storage/databases/main/events_worker.py
index 712b8ce204fe..2a255d103101 100644
--- a/synapse/storage/databases/main/events_worker.py
+++ b/synapse/storage/databases/main/events_worker.py
@@ -1854,7 +1854,7 @@ def is_event_next_to_gap_txn(txn: LoggingTransaction) -> bool:
             forward_edge_query = """
                 SELECT 1 FROM event_edges
                 /* Check to make sure the event referencing our event in question is not rejected */
-                LEFT JOIN rejections ON event_edges.event_id == rejections.event_id
+                LEFT JOIN rejections ON event_edges.event_id = rejections.event_id
                 WHERE
                     event_edges.room_id = ?
                     AND event_edges.prev_event_id = ?