I think it depends on the choices you set. For example, if you have a field called 'my_field' with choices:
'choices' => array('m' => 'Male', 'f' => 'Female')
and you set the post data with value 0, the field will be invalid because its value is not in the choices list.
http://symfony.com/doc/current/reference/forms/types/choice.html#choices