Undefined variable: captcha in my view

ahoy,

i'm trying to implement your component but i've a little issue.

here's my code on CommentController.php

class CommentsController extends AppController {

public $name = 'Comments';
public $helpers = array('Html', 'Form', 'Session');
public $components = array('MathCaptcha','RequestHandler');

public function add($post_id = null) {
            $this->set('captcha', $this->MathCaptcha->getCaptcha());
    if ($this->request->is('post')) {
        $this->Comment->set(array('post_id'=>$post_id));
        $this->request->data['Comment']['ip'] = $this->RequestHandler->getClientIp();

  if ($this->MathCaptcha->validate($this->request->data['Comment']['captcha'])) {
            $this->Session->setFlash("ok.","notif");
            $this->redirect(array('controller' => 'posts', 'action' => 'view', $post_id));
        } else {
             $this->Session->setFlash("ko.","notif");
        }
    }
}

}

and my view

Form->input('captcha', array('label' => 'Calculate this: '.$captcha)); ?>

but when i refresh i've an error who say:

Notice (8): Undefined variable: captcha [APP\View\Elements\newcomment.ctp, line 13]
(my line 13 is the line before

don't know why sadly ... can you please help me?