form submission

Form not sent if a select widget is left empty

Hi all,
I have a problem with a form. I have in it two select widgets. One is mandatory while the second not. So, it could be live empty by an user. Furthermore, the “submit” button has a control such as $form.$invalid.
Now, when I test the form it happens that after have made a selection in the first select widget (because it is mandatory), the button becomes active but the form is not sent (and so my task does not go on) until I select a value also on the second select widget. The strange thing that I saw is: