HMVC | Undefined property: $session | $this->session is null

[eluser]oukarai[/eluser]
in my login.php's validate_credentials action

Code:

function validate_credentials()
    {        
        $this->load->model('membership_model');
        
        $query = $this->membership_model->validate();

        if($query) { // if the user's credentials validated...
        
            $data = array(
                'username' => $this->input->post('username'),
                'is_logged_in' => true
            );
            
            $this->session->set_userdata($data);
            
            redirect('site/members_area');
            
        } else { // incorrect username or password
            
            $this->index();
            
        }
    }