angularjs checkbox onchange not working in form .$pristine

I have a form in angular and having some input boxes, checkbox and a submit button.Initially Submit button is Disabled. If I change any element from that form then the submit button become enable. I am making use of angularjs feature .$pristine and $invalid for that. It is working fine for input boxes but not working for checkbox change. Am I doing anything wrong? kindly guide me. See the fiddle :FIDDLE