diff --git a/synapse/storage/schema/main/delta/77/05thread_notifications_backfill.sql b/synapse/storage/schema/main/delta/77/05thread_notifications_backfill.sql
index ce6f9ff937..a5da7a17a0 100644
--- a/synapse/storage/schema/main/delta/77/05thread_notifications_backfill.sql
+++ b/synapse/storage/schema/main/delta/77/05thread_notifications_backfill.sql
@@ -21,7 +21,27 @@ DELETE FROM background_updates WHERE update_name = 'event_push_backfill_thread_i
-- Overwrite any null thread_id values.
UPDATE event_push_actions_staging SET thread_id = 'main' WHERE thread_id IS NULL;
UPDATE event_push_actions SET thread_id = 'main' WHERE thread_id IS NULL;
-UPDATE event_push_summary SET thread_id = 'main' WHERE thread_id IS NULL;
+
+-- Empirically we can end up with entries in the push summary table with both a
+-- `NULL` and `main` thread ID, which causes the insert below to fail. We fudge
+-- this by deleting any `NULL` rows that have a corresponding `main`.
+DELETE FROM event_push_summary AS a WHERE thread_id IS NULL AND EXISTS (
+ SELECT 1 FROM event_push_summary AS b
+ WHERE b.thread_id = 'main' AND a.user_id = b.user_id AND a.room_id = b.room_id
+);
+-- Copy the NULL threads to have a 'main' thread ID.
+--
+-- Note: Some people seem to have duplicate rows with a `NULL` thread ID, in
+-- which case we just fudge it with using MAX of the values. The counts *may* be
+-- wrong for such rooms, but a) its an edge case, and b) they'll be fixed when
+-- the user reads the room.
+INSERT INTO event_push_summary (user_id, room_id, notif_count, stream_ordering, unread_count, last_receipt_stream_ordering, thread_id)
+ SELECT user_id, room_id, MAX(notif_count), MAX(stream_ordering), MAX(unread_count), MAX(last_receipt_stream_ordering), 'main'
+ FROM event_push_summary
+ WHERE thread_id IS NULL
+ GROUP BY user_id, room_id, thread_id;
+
+DELETE FROM event_push_summary AS a WHERE thread_id IS NULL;
-- Drop the background updates to calculate the indexes used to find null thread_ids.
DELETE FROM background_updates WHERE update_name = 'event_push_actions_thread_id_null';
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