Magento 1.7 Admin Grid Problem

I have a magento admin grid module and when I am inside the grid on the admin menu it shows me the header and the add item button but it throws this error

Fatal error: Call to undefined method NAMESPACE_Garantiaextendida_Model_Mysql4_Garantiaextendida_Collection::setPageSize() in G:\xampp\htdocs\webpage\app\code\core\Mage\Adminhtml\Block\Widget\Grid.php on line 553

This is my mysql4 model collection

   <?php 

class Felisiakluciano_Garantiaextendida_Model_Mysql4_Garantiaextendida_Collection extends  Mage_Core_Model_Mysql4_Collection_Abstract
{
    protected function _construct()
    {
        $this->_init('garantiaextendida/garantiaextendida');
    }   
}

Can somebody help me? Please its more than urgent. Thanks and Good Bye.