how to load->library('image_lib') in MY_Upload extends CI_Upload

[eluser]Julia Yan[/eluser]
I had problem when load->library('image_lib') in MY_Upload extends CI_Upload

--------error message
Message: Undefined property: MY_Upload::$load

---------My class
class MY_Upload extends CI_Upload //CI_Upload
{

function __construct()
{
parent::CI_Upload(); //CI_Upload
}
function file_upload($params = array()) //works
{
...
}
function file_resize($params = array()) //
{
$this->load->library('image_lib');// Message: Undefined property: MY_Upload::$load
...
}

}