Session CI tidak jalan muncul tulisan "Unable to locate the specified class: Session.php"

Controller c_login:

 <?php
defined('BASEPATH') OR exit('No direct script access allowed');

class c_login extends CI_Controller {

public function index(){
	$this->load->view('v_login');
}

public function aksiLogin(){
	$user=$this->input->post('user');
	$password=$this->input->post('password');

	$data=array(
		'username'=>$user,
		'password'=>$password
		);

	$nilai=$this->m_penjualan->logindata('logindata',$data);
	if(count($nilai)>0){
		$datasession=array(
			'nama'=>$user,
			'status'=>"login"
			);
		$this->session->set_userdata($datasession);
		redirect('c_admin/index');
	}else{
		echo "Password dan username anda salah!!!";
	}
}

}

Model m_penjualan:

public function logindata($table,$where){
		return $this->db->get_where($table,$where);
	}

controller c_admin:

 <?php
defined('BASEPATH') OR exit('No direct script access allowed');

class c_admin extends CI_Controller {

	public function __construct()
	{
		parent::__construct();
		if($this->session->userdata('status') != "login"){
			redirect('c_login');
		}
	}

	public function index(){
		$this->load->view('v_admin');
	}
}