配置后台默认路由报 302重定向错误？ [ 2.0 版本 ]

The page isn’t redirecting properly
Firefox has detected that the server is redirecting the request for this address in a way that will never complete.

    'defaultRoute' => 'admin/login',    //配置默认路由

    public function actionLogin()
    {
        if (!Yii::$app->user->isGuest) {
            return $this->goHome();
        }
        $model = new LoginForm();
        if ($model->load(Yii::$app->request->post()) && $model->login()) {
            return $this->goBack();
        } else {
            return $this->renderPartial('login', [
                'model' => $model,
            ]);
        }
    }

设置默认为管理员登录页面, 奇怪了换成别的路由却能用？ 求解释