diff --git a/migrations/2025-03-17-110023_site_person_ban/up.sql b/migrations/2025-03-17-110023_site_person_ban/up.sql
index 7f87f509d..f35a19da5 100644
--- a/migrations/2025-03-17-110023_site_person_ban/up.sql
+++ b/migrations/2025-03-17-110023_site_person_ban/up.sql
@@ -5,7 +5,20 @@ ALTER TABLE instance_actions
     ADD COLUMN ban_expires timestamptz;
 
 ALTER TABLE mod_ban
-    ADD COLUMN instance_id int NOT NULL REFERENCES instance ON UPDATE CASCADE ON DELETE CASCADE;
+    ADD COLUMN instance_id int REFERENCES instance ON UPDATE CASCADE ON DELETE CASCADE;
+
+UPDATE
+    mod_ban
+SET
+    instance_id = person.instance_id
+FROM
+    person
+WHERE
+    mod_ban.instance_id IS NULL
+    AND mod_ban.mod_person_id = person.id;
+
+ALTER TABLE mod_ban
+    ALTER COLUMN instance_id SET NOT NULL;
 
 -- insert existing bans into instance_actions table, assuming they were all banned from home instance
 INSERT INTO instance_actions (person_id, instance_id, received_ban, ban_expires)