diff --git a/packages/backend/src/server/ActivityPubServerService.ts b/packages/backend/src/server/ActivityPubServerService.ts
index a0ad4a134f0c7688d42988101374df9d5d67c133..967a69cb77760b3755ee337650e2dd829dc84cd9 100644
--- a/packages/backend/src/server/ActivityPubServerService.ts
+++ b/packages/backend/src/server/ActivityPubServerService.ts
@@ -131,6 +131,7 @@ export class ActivityPubServerService {
 			const instanceActor = await this.instanceActorService.getInstanceActor();
 
 			if (userId === instanceActor.id) return false;
+			if (userId === instanceActor.username) return false;
 		}
 
 		let signature;