如何解决此“在布尔值上调用成员函数row（）”错误？

我想从mysql获取一些数据。我为此使用codeigniter模型和控制器。

CI_model.php：

     public function getLastSale($id){


     $q = $this->db->query("SELECT * from sma_sales  desc limit 1 where customer_id = '$id' ");


    $result = $q->row();


    $res = array();
    $res['id'] = $result->id;
    $res['paid'] = $result->paid;


    return $res; 


}

CI_controller.php：

    $getLastData = $this->pos_model->getLastSale($customer_id);

    $sid = $getLastData['id'];

    $prepaid = $getLastData['paid'];

但是此错误显示：

    An uncaught Exception was encountered
    Type: Error

    Message: Call to a member function row() on boolean

我是codeigniter的初学者。我究竟做错了什么 ？