ng-model on <input type='checkbox' doesn't set class 'ng-dirty'

[zoechi]

I have a checkbox (<input type='checkbox' ng-model='ctrl.someState'>) where the class 'ng-pristine' is set initially.
When I check/uncheck the checkbox 'ng-pristine' should be updated to 'ng-dirty'
(I hope my assumptions about this are right - it works for text, number, url, email though)
but the class doesn't change.

The model value (ctrl.someState) gets changed.