diff --git a/packages/backend/src/server/api/endpoints/i/update.ts b/packages/backend/src/server/api/endpoints/i/update.ts
index 20ccfd5e8..31af07cfb 100644
--- a/packages/backend/src/server/api/endpoints/i/update.ts
+++ b/packages/backend/src/server/api/endpoints/i/update.ts
@@ -91,7 +91,7 @@ export const paramDef = {
 		birthday: { ...Users.birthdaySchema, nullable: true },
 		lang: {
 			type: "string",
-			enum: Object.keys(langmap),
+			enum: [null, ...Object.keys(langmap)],
 			nullable: true,
 		},
 		avatarId: { type: "string", format: "misskey:id", nullable: true },
@@ -159,7 +159,7 @@ export default define(meta, paramDef, async (ps, _user, token) => {
 
 	if (ps.name !== undefined) updates.name = ps.name;
 	if (ps.description !== undefined) profileUpdates.description = ps.description;
-	if (typeof ps.lang === "string") profileUpdates.lang = ps.lang;
+	if (ps.lang !== undefined) profileUpdates.lang = ps.lang;
 	if (ps.location !== undefined) profileUpdates.location = ps.location;
 	if (ps.birthday !== undefined) profileUpdates.birthday = ps.birthday;
 	if (ps.ffVisibility !== undefined)