Dingo API 在response->collection()返回空值

dingo/api的response->collection方法是有什么特殊的设置么？
下面的代码，当$domains查数据库返回数据时，得到的body是空的，但是http_code是200。

问题截图：

Laravel 5.6
dingo dev-master
jwt-auth dev-master
PHP 7.1

Controller.php
``

public function GetByMethod($method,$data)
{
    $transformer = [
        'domain_id' => 'id',
        'domain_name' => 'name',
        'master_server' => 'master',
        'last_check_time' => 'last_check',
        'zone_type' => 'type',
        'maintenance_account' => 'account'
    ];
    if(array_key_exists($method,$transformer))
    {
        $domains = Domain::where($transformer[$method],'=',$data)->get();
        if (count($domains))
        {
            $this->response->collection($domains,new DomainTransformer());
        }
        else
        {
            $this->response->errorNotFound();
        }
    }
    else
    {
        $this->response->errorBadRequest('Invalid query method');
    }
}

``

DomainTransformer.php
``
class DomainTransformer extends TransformerAbstract
{

public function transform(Domain $domain)
{
    return [
        'domain_id' => $domain['id'],
        'domain_name' => $domain['name'],
        'master_server' => $domain['master'],
        'last_check_time' => $domain['last_check'],
        'zone_type' => $domain['type'],
        'maintenance_account' => $domain['account']
    ];
}

}
``