diff --git a/CHANGES.md b/CHANGES.md
index 905713b2af..fbcacab5bc 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,3 +1,14 @@
+Synapse 1.85.1 (2023-06-07)
+===========================
+
+Note: this release only fixes a bug that stopped some deployments from upgrading to v1.85.0. There is no need to upgrade to v1.85.1 if successfully running v1.85.0.
+
+Bugfixes
+--------
+
+- Fix bug in schema delta that broke upgrades for some deployments. Introduced in v1.85.0. ([\#15738](https://github.com/matrix-org/synapse/issues/15738), [\#15739](https://github.com/matrix-org/synapse/issues/15739))
+
+
 Synapse 1.85.0 (2023-06-06)
 ===========================
 
diff --git a/debian/changelog b/debian/changelog
index 2278a83283..6d6f10ddf1 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -1,3 +1,9 @@
+matrix-synapse-py3 (1.85.1) stable; urgency=medium
+
+  * New Synapse release 1.85.1.
+
+ -- Synapse Packaging team <packages@matrix.org>  Wed, 07 Jun 2023 10:51:12 +0100
+
 matrix-synapse-py3 (1.85.0) stable; urgency=medium
 
   * New Synapse release 1.85.0.
diff --git a/pyproject.toml b/pyproject.toml
index 434eaa52ae..d9ce1d84d6 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -89,7 +89,7 @@ manifest-path = "rust/Cargo.toml"
 
 [tool.poetry]
 name = "matrix-synapse"
-version = "1.85.0"
+version = "1.85.1"
 description = "Homeserver for the Matrix decentralised comms protocol"
 authors = ["Matrix.org Team and Contributors <packages@matrix.org>"]
 license = "Apache-2.0"
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';