codeigniter会话变为值'css'

Question

I have a very strange thing going on. The value of my session always becomes 'css' after a while. Can you guys please have a look?

public function index($step = 13)
{
    if(!$this->session->userdata('laststep') && $step != 13)
    {
        $this->session->set_userdata('laststep', $step);
    }
    if($step > $this->session->userdata('laststep') && $step != 13) 
    {
        $this->session->set_userdata('laststep', $step);
    }
    if($step < $this->session->userdata('laststep'))    
    {
        redirect(base_url().'main/index/'.$this->session->userdata('laststep'));
    }
}

every time, the link goes 1 step up. It goes fine for 2 clicks or something and then I come in the 3rd if scenario and being redirected to /main/index/css

Where the hell comes that 'css' from. How can the value of my session change into that?