Issue With Radio Buttons

I have a view using $form as CActiveForm and I render two radio buttons:

echo $form->radioButton($model, 'domain', array('id' => 'rad1', 'value' => 'value1'));

here is value1

echo $form->radioButton($model, 'domain', array('id' => 'rad2', 'value' => 'value2'));

here is value2

When the form is submitted and the first radio button is selected, instead of receiving the value value1, I get 0 instead. Checking the second radio button works fine and sends value2.

Why is this happening?