set a rule on checkbox

Hi,

I would like that the user is obliged to tick one of the checkbox. Here's
the code for adding checkbox :

while ($do->fetch()) {
$form->addElement('checkbox', 'addmultiple[' . $do->profcode. ']', ' '
,$do->proflibel, array('checked' => 'checked'));
}


I thought that it was enough to add a rule like this :

$form->addRule('addmultiple','profil obligatoire', 'required', null,
'client');

but that does not work !