form_dropdown() return undefined

[eluser]Zied[/eluser]
Is the same problem, even when I used <select> with its parameters

Code:

$options = array(
                  'comment'  => $contact_comment,
                  'suggestion'  => $contact_suggestion,
                  'reclamation'  => $contact_reclamation,
                );
// all this form_dropdown is failed
echo form_dropdown('subject', $options,'id="comment"');
echo form_dropdown('subject', $options,'comment');
echo form_dropdown('subject', $options);
echo form_dropdown('id="subject"', $options,'comment');

I tested the sending of data with ajax, and I think it works
I even tried the controller and it works fine
I'm sure the problem in the <select> or also form_dropdown.
But I do not know how to fix it.