diff --git a/packages/backend/migration/1748128176881-add_instance_foreign_keys.js b/packages/backend/migration/1748128176881-add_instance_foreign_keys.js
index da2de14f18..9a970699a0 100644
--- a/packages/backend/migration/1748128176881-add_instance_foreign_keys.js
+++ b/packages/backend/migration/1748128176881-add_instance_foreign_keys.js
@@ -15,6 +15,11 @@ export class AddInstanceForeignKeys1748128176881 {
 	name = 'AddInstanceForeignKeys1748128176881'
 
 	async up(queryRunner) {
+		// Fix-up: Sharkey versions in early-mid 2025 could federate with HTTP URLs, which would produce a user with no matching instance.
+		// These users are fundamentally broken and can just be removed, which ensures that the FK can create without conflicts.
+		// But we must also *preserve* those with a matching registered instance, as FireFish allowed federation over HTTP and some older instances may have fully-populated users.
+		await queryRunner.query(`DELETE FROM "user" WHERE "uri" LIKE 'http:%' AND NOT EXISTS (select 1 from "instance" where "instance"."host" = "user"."host")`);
+
 		await queryRunner.query(`ALTER TABLE "user" ADD CONSTRAINT "FK_user_host" FOREIGN KEY ("host") REFERENCES "instance"("host") ON DELETE CASCADE ON UPDATE NO ACTION`);
 		await queryRunner.query(`ALTER TABLE "note" ADD CONSTRAINT "FK_note_userHost" FOREIGN KEY ("userHost") REFERENCES "instance"("host") ON DELETE CASCADE ON UPDATE NO ACTION`);
 		await queryRunner.query(`ALTER TABLE "note" ADD CONSTRAINT "FK_note_replyUserHost" FOREIGN KEY ("replyUserHost") REFERENCES "instance"("host") ON DELETE CASCADE ON UPDATE NO ACTION`);