[fw-general] problem with Zend_Form_Element_Captcha

[PontoSI - Consultoria, Informática e Serviços LDA]

Hi,
I've made a form with several fields and a Zend_Form_Element_Captcha 
element (image based). When the form is submitted (via POST)
using the submit button, if the captcha textbox is empty it will show 
the badCaptcha message as it is supposed to, but if
I hit reload on the browser and confirm resubmitting the form the 
captcha is magically bypassed and it is executed the
code inside the $frm->isValid() block.
Can anyone who is using Zend_Form_Element_Captcha  try to reproduce the 
error and/or check if I am missing something obvious?
A simplified version of the code I'm using follows below.
Regards,
   João Pinheiro


/* Form Class */
class AForm extends Zend_Form {

   public function __construct($action = '') {
       global $Settings;

       parent::__construct();

       $this->setAction($action)
            ->setName('frms')
            ->setMethod('post');

      // [...] other elements removed

      $params = array(
           'label' => 'Introduza o código:',
           'required' => true,
           'captcha' => 'Image',
           'captchaOptions' => array(
               'captcha' => 'Image',
               'wordLen' => 6,
               'timeout' => 300,
               'font'    =>  $Settings->parameters->path . 
'/fonts/FreeSerif.ttf',
               'imgDir' => $Settings->parameters->rootpath . '/captcha/',
               'imgUrl' => $Settings->parameters->url . '/captcha/',
               'height' => 40
           ));
       $this->addElement('captcha', 'cid', $params);   
}

(...)
/* Action inside controller */
   public function formAction() {
       global $Settings;
      
       $frm = new AForm($this->Settings->parameters . 
$this->actionPath('form'));
       $this->view->form = $frm;
       $this->view->submitted = false;
      
       if ($this->getRequest()->isPost()) {
           if ($frm->isValid($_POST)) {
               $this->view->submitted = true;
               /* $frm->send(); */
           }
       }
       $this->renderToBody('form.phtml');
   }