diff --git a/packages/backend/src/core/activitypub/ApRequestService.ts b/packages/backend/src/core/activitypub/ApRequestService.ts
index eb9059ba0911..7cf8359212c1 100644
--- a/packages/backend/src/core/activitypub/ApRequestService.ts
+++ b/packages/backend/src/core/activitypub/ApRequestService.ts
@@ -205,7 +205,7 @@ export class ApRequestService {
 		//#region リクエスト先がhtmlかつactivity+jsonへのalternate linkタグがあるとき
 		const contentType = res.headers.get('content-type');
 
-		if (contentType?.split(';')[0]?.trimEnd()?.toLowerCase() === 'text/html' && _followAlternate === true) {
+		if ((contentType ?? '').split(';')[0].trimEnd().toLowerCase() === 'text/html' && _followAlternate === true) {
 			const html = await res.text();
 			const window = new Window();
 			const document = window.document;