::find() and findFirst() return null, why? help!!!

my table column is:

| id | typesId | name | price | quantity | status |

my model code is:

class Products extends \Phalcon\Mvc\Model {

public $id; 
public $typesId;
public $name;
public $price;  
public $quantity;
public $status;

public function getSource()
{
    return 'products';
}
public static function find($parameters = null)
{
    return parent::find($parameters);
}
public static function findFirst($parameters = null)
{
    return parent::findFirst($parameters);
}

}

when i use this code:

$data = Products::find();

var_dump($data);

the variable $data was NULL, why？

I need help，thank you

PHP VERSION: 5.6.9 phalcon version : 2.0.13