diff --git a/packages/backend/src/core/activitypub/models/ApPersonService.ts b/packages/backend/src/core/activitypub/models/ApPersonService.ts
index 398c8695d2eb..0ec07eb141e4 100644
--- a/packages/backend/src/core/activitypub/models/ApPersonService.ts
+++ b/packages/backend/src/core/activitypub/models/ApPersonService.ts
@@ -20,7 +20,7 @@ import type Logger from '@/logger.js';
 import type { MiNote } from '@/models/Note.js';
 import type { IdService } from '@/core/IdService.js';
 import type { MfmService } from '@/core/MfmService.js';
-import { toArray } from '@/misc/prelude/array.js';
+import { toArray, toSingle } from '@/misc/prelude/array.js';
 import type { GlobalEventService } from '@/core/GlobalEventService.js';
 import type { FederatedInstanceService } from '@/core/FederatedInstanceService.js';
 import type { FetchInstanceMetadataService } from '@/core/FetchInstanceMetadataService.js';
@@ -181,6 +181,7 @@ export class ApPersonService implements OnModuleInit {
 			throw new Error('invalid Actor: id has different host');
 		}
 
+		x.publicKey = toSingle(x.publicKey);
 		if (x.publicKey) {
 			if (typeof x.publicKey.id !== 'string') {
 				throw new Error('invalid Actor: publicKey.id is not a string');