diff --git a/packages/backend/src/core/WebhookTestService.ts b/packages/backend/src/core/WebhookTestService.ts
index c4b01d535b..8dc42e45c0 100644
--- a/packages/backend/src/core/WebhookTestService.ts
+++ b/packages/backend/src/core/WebhookTestService.ts
@@ -13,6 +13,7 @@ import { type WebhookEventTypes } from '@/models/Webhook.js';
 import { CustomEmojiService } from '@/core/CustomEmojiService.js';
 import { type UserWebhookPayload, UserWebhookService } from '@/core/UserWebhookService.js';
 import { QueueService } from '@/core/QueueService.js';
+import { IdService } from '@/core/IdService.js';
 import { ModeratorInactivityRemainingTime } from '@/queue/processors/CheckModeratorsActivityProcessorService.js';
 
 const oneDayMillis = 24 * 60 * 60 * 1000;
@@ -166,6 +167,7 @@ export class WebhookTestService {
 		private userWebhookService: UserWebhookService,
 		private systemWebhookService: SystemWebhookService,
 		private queueService: QueueService,
+		private readonly idService: IdService,
 	) {
 	}
 
@@ -449,6 +451,8 @@ export class WebhookTestService {
 				offsetX: it.offsetX,
 				offsetY: it.offsetY,
 			})),
+			createdAt: this.idService.parse(user.id).date.toISOString(),
+			description: '',
 			isBot: user.isBot,
 			isCat: user.isCat,
 			emojis: await this.customEmojiService.populateEmojis(user.emojis, user.host),
diff --git a/packages/backend/test/utils.ts b/packages/backend/test/utils.ts
index 5da5353e09..52cfb8ac93 100644
--- a/packages/backend/test/utils.ts
+++ b/packages/backend/test/utils.ts
@@ -652,7 +652,7 @@ export async function sendEnvResetRequest() {
 
 // 与えられた値を強制的にエラーとみなす。この関数は型安全性を破壊するため、異常系のアサーション以外で用いられるべきではない。
 // FIXME(misskey-js): misskey-jsがエラー情報を公開するようになったらこの関数を廃止する
-export function castAsError(obj: Record<string, unknown>): { error: ApiError } {
+export function castAsError(obj: object | null | undefined): { error: ApiError } {
 	return obj as { error: ApiError };
 }