var router = new require('koa-router');
router.use(function *(next) {
    // first middleware, do somethins
    yield next;
});
router.use('/test', function *(next) {
    // second middleware, do somethins
    yield next;
});

router.use(function *(next) {
    // thrid middleware, do somethins
    yield next;
});

router.all('/', function () {
    this.body = "hello world";
});

当我请求 /的时候，在第三个中间件抛出异常，next is undefined，这该怎么处理