diff --git a/packages/backend/test/e2e/endpoints.ts b/packages/backend/test/e2e/endpoints.ts
index 2b2699ecd9..791a33e985 100644
--- a/packages/backend/test/e2e/endpoints.ts
+++ b/packages/backend/test/e2e/endpoints.ts
@@ -10,7 +10,7 @@ import * as assert from 'assert';
 // https://github.com/node-fetch/node-fetch/pull/1664
 import { Blob } from 'node-fetch';
 import { MiUser } from '@/models/_.js';
-import { api, initTestDb, post, signup, simpleGet, uploadFile } from '../utils.js';
+import {api, castAsError, initTestDb, post, signup, simpleGet, uploadFile} from '../utils.js';
 import type * as misskey from 'misskey-js';
 
 describe('Endpoints', () => {
@@ -285,7 +285,8 @@ describe('Endpoints', () => {
 			}, alice);
 
 			assert.strictEqual(res.status, 400);
-			assert.strictEqual(res.body.error.code, 'CANNOT_REACT_TO_RENOTE');
+			assert.ok(res.body);
+			assert.strictEqual(castAsError(res.body).error.code, 'CANNOT_REACT_TO_RENOTE');
 		});
 
 		test('引用にリアクションできる', async () => {