如何验证表单集合

I'm not being able to validate a form Collection element, here is how my fieldset looks like:

class ProductFieldset extends Fieldset implements InputFilterProviderInterface
{
    public function init()
    {
        $this->setName('product');

        $this->add(array(
            'name' => 'features',
            'type' => 'Collection',
            'options' => array(
                'label'          => 'Características do produto',
                'count'          => 0,
                'should_create_template' => true,
                'allow_add'      => true,
                'target_element' => array(
                    'type'    => 'Text',
                ),
            ),
        ));
    }

    public function getInputFilterSpecification()
    {
        return array(
            'features' => array(
                'required' => false,
                'filters' => array(
                    array('name' => 'StringTrim'),
                ),
                'validators' => array(
                    array(
                        'name' => 'StringLength',
                        'options' => array(
                            'min'      => 10,
                        ),
                    ),
                ),
            ),
        );
    }
}

If I add an element and leave it empty the validation passes, but it should not, since StringLength requires at least 10 characters. I must be missing something, if someone can show me what I'm doing wrong I would be very thankful.