diff --git a/packages/backend/src/core/activitypub/models/ApPersonService.ts b/packages/backend/src/core/activitypub/models/ApPersonService.ts
index c9de67b3a006..8c4e40c561c6 100644
--- a/packages/backend/src/core/activitypub/models/ApPersonService.ts
+++ b/packages/backend/src/core/activitypub/models/ApPersonService.ts
@@ -355,7 +355,7 @@ export class ApPersonService implements OnModuleInit {
 					usernameLower: person.preferredUsername?.toLowerCase(),
 					host,
 					inbox: person.inbox,
-					sharedInbox: person.sharedInbox ?? person.endpoints?.sharedInbox,
+					sharedInbox: person.sharedInbox ?? person.endpoints?.sharedInbox ?? null,
 					followersUri: person.followers ? getApId(person.followers) : undefined,
 					featured: person.featured ? getApId(person.featured) : undefined,
 					uri: person.id,
@@ -521,7 +521,7 @@ export class ApPersonService implements OnModuleInit {
 		const updates = {
 			lastFetchedAt: new Date(),
 			inbox: person.inbox,
-			sharedInbox: person.sharedInbox ?? person.endpoints?.sharedInbox,
+			sharedInbox: person.sharedInbox ?? person.endpoints?.sharedInbox ?? null,
 			followersUri: person.followers ? getApId(person.followers) : undefined,
 			featured: person.featured,
 			emojis: emojiNames,
@@ -593,7 +593,7 @@ export class ApPersonService implements OnModuleInit {
 		// 該当ユーザーが既にフォロワーになっていた場合はFollowingもアップデートする
 		await this.followingsRepository.update(
 			{ followerId: exist.id },
-			{ followerSharedInbox: person.sharedInbox ?? person.endpoints?.sharedInbox },
+			{ followerSharedInbox: person.sharedInbox ?? person.endpoints?.sharedInbox ?? null },
 		);
 
 		await this.updateFeatured(exist.id, resolver).catch(err => this.logger.error(err));