[fw-core] Zend_Form_Element_Captcha doesn't works as expected !

Hello,

I'm trying to create a captcha with Image.
It practically works excepts that I see the image and 2 fields ...

I'm using ZF 1.8.0

This the generated code :

<div class="field"><label for="captcha" class="required">are_you_human</label>

<img alt="" src="/share/img/captchas/631567244e675bdf5ec0abbb61b6fe58.png"/><br/>
<input type="hidden" name="captcha[id]" value="631567244e675bdf5ec0abbb61b6fe58" helper="formText" id="captcha-id">
<input type="text" name="captcha[input]" id="captcha-input" value="" helper="formText">
<input type="text" name="captcha" id="captcha" value="631567244e675bdf5ec0abbb61b6fe58"></div>

As you can see Zend_Form render a third field ... Why ????
And in my Zend_Form class I initialize the element like this

$elt = new Zend_Form_Element_Captcha('captcha', array(
'label' => "are_you_human",
'captcha' => array(
'captcha' => 'Image',
'wordLen' => 6,
'timeout' => 300,
'font' => './share/fonts/verdana.ttf',
'imgDir' => './share/img/captchas/',
'imgUrl' => '/share/img/captchas/',
)
));

$this->addElement($elt);

What I've missed ... It's making me crazy
Thank you for you help

Regards