sfWidgetFormChoice有多个选择

I'm using symfony 1.4 to make a CRUD from a client. This client need to have a select field with multiple choices to generate a newsletter and sent an email. I'm setting the sfWidgetFormChoice with multiple => 'true', but always i can select just one of items on select. I don't know how to fix this, the googling don't help me. Look at my code:

$this->widgetSchema['id_news'] = new sfWidgetFormChoice(array('label' => 'News',
    "choices" => $this->getNewsLimitUm(),
    'multiple' => true,
 ));

$this->validatorSchema['id_noticia'] = new sfValidatorString(array('required' => false));

The problem is with my Id, from select field. I change it and work fine.