diff --git a/changelog.d/7688.bugfix b/changelog.d/7688.bugfix
new file mode 100644
index 000000000000..2ed718a08791
--- /dev/null
+++ b/changelog.d/7688.bugfix
@@ -0,0 +1 @@
+Fix "Starting db txn 'get_completed_ui_auth_stages' from sentinel context" warning. The bug was introduced in 1.13.0rc1.
diff --git a/synapse/storage/data_stores/main/ui_auth.py b/synapse/storage/data_stores/main/ui_auth.py
index 1d8ee22fb117..ec2f38c37357 100644
--- a/synapse/storage/data_stores/main/ui_auth.py
+++ b/synapse/storage/data_stores/main/ui_auth.py
@@ -186,7 +186,7 @@ async def set_ui_auth_clientdict(
         # The clientdict gets stored as JSON.
         clientdict_json = json.dumps(clientdict)
 
-        self.db.simple_update_one(
+        await self.db.simple_update_one(
             table="ui_auth_sessions",
             keyvalues={"session_id": session_id},
             updatevalues={"clientdict": clientdict_json},