[fw-mvc] How to add errors to subForms?

Hi all,
I have a  form with many subForms and these subForms have many subForms too, sometimes.
But now, I need add a custom error on subForm element.

I'm trying:

$form->getSubForm('Person')
                     ->getSubForm('PersonDocument')
                     ->getElement('cpf')
                     ->addError('Como a pessoa é Brasileira, o CPF deve ser obrigatório');

But it dont works...

Someone knows how do it?

Best Regs,
______________________________________
Daniel Lima
Web Developer
Tel.: (33) 3536 1091
Cel.: (33) 9139 5521
Skype: yourwebmaker
Cilens Software
http://www.cilens.info/