diff --git a/packages/backend/src/server/api/endpoints/i/update.ts b/packages/backend/src/server/api/endpoints/i/update.ts
index c438a33c..2fd10ede 100644
--- a/packages/backend/src/server/api/endpoints/i/update.ts
+++ b/packages/backend/src/server/api/endpoints/i/update.ts
@@ -90,7 +90,7 @@ export const paramDef = {
 		birthday: { ...Users.birthdaySchema, nullable: true },
 		lang: {
 			type: "string",
-			enum: Object.keys(langmap),
+			enum: [...Object.keys(langmap), null],
 			nullable: true,
 		},
 		avatarId: { type: "string", format: "misskey:id", nullable: true },