diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1b0577fe32b..e8a07b416c1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -25,6 +25,7 @@
 - Enhance: リモートノートクリーニングジョブの削除対象検索処理のパフォーマンス改善
 - Fix: センシティブメディア自動検出周りの依存関係・ファイルの解決に失敗する問題を修正
 - Fix: フォロワー限定投稿を指名投稿で引用した際に、引用した投稿の公開範囲が意図せず変更される問題を修正
+- Fix: `actor` を持たないInboxアクティビティを受信した際に `getApId()` が `TypeError` を投げて配送ジョブがクラッシュする問題を修正
 
 ## 2026.5.4
 
diff --git a/packages/backend/src/core/activitypub/type.ts b/packages/backend/src/core/activitypub/type.ts
index 72732b01dfe..be867156275 100644
--- a/packages/backend/src/core/activitypub/type.ts
+++ b/packages/backend/src/core/activitypub/type.ts
@@ -58,8 +58,8 @@ export function getOneApId(value: ApObject): string {
  */
 export function getApId(value: string | IObject): string {
 	if (typeof value === 'string') return value;
-	if (typeof value.id === 'string') return value.id;
-	throw new Error('cannot detemine id');
+	if (value != null && typeof value.id === 'string') return value.id;
+	throw new Error('cannot determine id');
 }
 
 /**
diff --git a/packages/backend/src/queue/processors/InboxProcessorService.ts b/packages/backend/src/queue/processors/InboxProcessorService.ts
index b7557ddb6f9..7c2d3c22cfe 100644
--- a/packages/backend/src/queue/processors/InboxProcessorService.ts
+++ b/packages/backend/src/queue/processors/InboxProcessorService.ts
@@ -73,6 +73,12 @@ export class InboxProcessorService implements OnApplicationShutdown {
 		this.logger.debug(JSON.stringify(info, null, 2));
 		//#endregion
 
+		// Activities without an actor are invalid; skip them cleanly instead of
+		// throwing a TypeError deep inside getApId(activity.actor).
+		if (activity.actor == null) {
+			throw new Bull.UnrecoverableError('skip: activity has no actor');
+		}
+
 		const host = this.utilityService.toPuny(new URL(signature.keyId).hostname);
 
 		if (!this.utilityService.isFederationAllowedHost(host)) {