help me about load session

my __construct() on subfolder controller

PHP Code:

public function __construct() {
    parent::__construct();
    $this->load->model('backend/Landingexpress_model', 'Landingexpress_model');
} 


config session

PHP Code:

$autoload['libraries'] = array('database','upload', 'session');
///
$config['encryption_key'] = 'asdhkjhdfuwehrljkhwejrh';
///
$config['sess_driver'] = 'database';
$config['sess_cookie_name'] = 'ci_session';
$config['sess_expiration'] = 7200;
$config['sess_save_path'] = 'ci_sessions';
$config['sess_match_ip'] = FALSE;
$config['sess_time_to_update'] = 300;
$config['sess_regenerate_destroy'] = FALSE; 


when i load model on __construct() my view is error 'Unable to locate the specified class: Session.php';

when i not load model session is working.

i dont where i wrong thank you.