diff --git a/tildes/tildes/views/api/web/comment.py b/tildes/tildes/views/api/web/comment.py
index 647e52c..b7d6b54 100644
--- a/tildes/tildes/views/api/web/comment.py
+++ b/tildes/tildes/views/api/web/comment.py
@@ -348,6 +348,11 @@ def put_mark_comments_read(
             .all()
         )
 
+        # sort the notifications by created_time of their comment so that the
+        # INSERT ... ON CONFLICT DO UPDATE statements work as expected
+        prev_notifications = sorted(
+            prev_notifications, key=lambda c: c.comment.created_time)
+
         for comment_notification in prev_notifications:
             comment_notification.is_unread = False
             _increment_topic_comments_seen(