Why is setting a checkbox's false value to 'undefined' causing ng-invalid to get added

I have a checkbox in angular that has a false value of undefined and despite it not being required, my form does not validate when it is checked and unchecked, due to ng-invalid being added as a class to the input on uncheck.

Why does this happen?

Here is a plunker you can mess with: http://ift.tt/1JySkIg

Notice the second checkbox has a false value of 'undefined'.