diff --git a/packages/backend/src/core/activitypub/ApRendererService.ts b/packages/backend/src/core/activitypub/ApRendererService.ts
index 77c5f207f8..623e7002cd 100644
--- a/packages/backend/src/core/activitypub/ApRendererService.ts
+++ b/packages/backend/src/core/activitypub/ApRendererService.ts
@@ -545,7 +545,7 @@ export class ApRendererService {
 			attributedTo,
 			summary: summary ?? undefined,
 			content: content ?? undefined,
-			updated: note.updatedAt?.toISOString(),
+			updated: note.updatedAt?.toISOString() ?? undefined,
 			_misskey_content: text,
 			source: {
 				content: text,