Unable to locate the specified class: Session.php in Codigniter 3

Question

Adding a hook as for pre_controller gives following error: Unable to locate the specified class: Session.php

hook.php:

$hook['pre_controller'][] = array(
    'class' => 'Load_constants',
    'function' => 'index',
    'filename' => 'Load_constants.php',
    'filepath' => 'hooks',
    'params' => array()
);

Load_constants.php:

class Load_constants extends CI_Controller {
    function __construct(){
        parent::__construct();
    }
.....

If I remove extended CI_Controller it works well and adding it will throw above error

This issue just appear in CI3 older version works well