diff --git a/src/common/middlewares/upload.middleware.ts b/src/common/middlewares/upload.middleware.ts
index 0ea5565..48f5ae1 100644
--- a/src/common/middlewares/upload.middleware.ts
+++ b/src/common/middlewares/upload.middleware.ts
@@ -53,15 +53,16 @@ export class UploadMiddleware implements NestMiddleware {
 
   use(req: Request, _res: Response, next: NextFunction): void {
     // @types/express 与 @types/multer 内嵌的 @types/express-serve-static-core 版本不一致，
-    // 运行时完全等价，这里用一次类型断言绕过编译期差异
-    const single = this.upload.single('file') as (
+    // 运行时完全等价，用 unknown 中转两次绕过编译期差异
+    const single = this.upload.single('file') as unknown as (
       req: Request,
       res: Response,
       cb: (err: unknown) => void,
     ) => void;
     single(req, _res, (err) => {
       if (err) {
-        next(new BadRequestException(err.message ?? '文件上传失败'));
+        const msg = err instanceof Error ? err.message : '文件上传失败';
+        next(new BadRequestException(msg));
         return;
       }
       next();