diff --git a/packages/backend/src/server/web/boot.js b/packages/backend/src/server/web/boot.js
index c6cb25e43a..887a4c8e01 100644
--- a/packages/backend/src/server/web/boot.js
+++ b/packages/backend/src/server/web/boot.js
@@ -61,6 +61,10 @@
 			renderError('META_FETCH_V');
 			return;
 		}
+
+		// for https://github.com/misskey-dev/misskey/issues/10202
+		if (lang == null || lang.toString == null || lang.toString() === 'null') lang = 'en-US';
+
 		const localRes = await window.fetch(`/assets/locales/${lang}.${v}.json`);
 		if (localRes.status === 200) {
 			localStorage.setItem('lang', lang);