CodeIgniter 2 can't extend CI_Model

[eluser]John_Betong[/eluser]
I have my model in the ./application/model/m_lib path and use autoload.php

 
File: ./application/config/autoload.php

Code:

/*
| -------------------------------------------------------------------
|  Auto-load Models
| -------------------------------------------------------------------
| Prototype:
|
|    $autoload['model'] = array('model1', 'model2');
|
*/
$autoload['model'] = array('m_lib');