symfony2: selected value of choices fields

I get something strange with Symfony2 forms. I create a form with a propel entity, values are fine except the "select" (choices) field, that have no selected value.

I tried few tricks like:

$params['choices'] = array('N/A'=> 'N/A');
$params['data'] = array('N/A');
$params['preferred_choices'] = array('N/A');

Even with this, there is no preselected value. What's wrong ?