[fw-mvc] Zend_Form_Element_Submit class attribute

How to add a 'class' attribute to Zend_Form_Element_Submit element? It
doesn't recognize it whatever way I tried:

$this->addElement(
'Submit',
'btn_search',
array(
'label' => 'Search',
'decorators' => $this->buttonDecorators,
'class' => 'class_name'
)
);

or
$this->getElement('btn_search')->setAttrib('class', 'class_name');