Zend_Form_Element_Multiselect and option with key '0'

Example code:

        $categories = new Zend_Form_Element_Multiselect('categories');
        $categories->setLabel('Родительская категория');
        $categories->addMultiOptions(array(
        '0' => 'Root',
            '1' => 'One more',
        ));
        $categories->size = 5;

        $form = new Zend_Form();
        $form->addElement($categories);
        $form->populate(array('categories' => '0'));

        echo $form;

First option (with key '0') is not marked 'selected'...