So I have a form that is using the Zend_Filter_Null filter. When I call it directly, it works:

$makeZeroNull = new Zend_Filter_Null();
$null = $makeZeroNull->filter('0');
//$null === null

However, when I try to add it to an element in my form, it doesn't filter the value when I call getValue().

class My_Form extends Zend_Form {
    public function init() {
        $makeZeroNull = new Zend_Filter_Null();
        $this->addElement('text', 'State_ID', array('filters' => array($makeZeroNull)));
    }
}

//in controller
if ($form->isValid($_POST)) {
    $zero = $form->State_ID->getValue();
    //getValue() should return null, but it is returning 0
}

What is going on? What am I doing wrong?