simple angular checkboxes

I have a T/F property called "valid" returning from an API call. I want to accurately display it as a checkbox, as well as allow the user to set/unset it. Setting or unsetting it will make a SAVE call.

<md-checkbox
      aria-label="Confirmed"
      ng-model="vm.selectedQuestion.valid" 
      ng-click="vm.setReviewed()" 
      ng-checked="vm.selectedQuestion.valid">
       Valid
</md-checkbox>

.

        vm.setReviewed = function () {
            vm.selectedQuestion.valid = !vm.selectedQuestion.valid;
            // a bunch of other stuff
        };

If the question gets loaded with value: true, then I see the checked box. If I uncheck the box, the valid property disappears from the object completely.

I know I'm doing something wrong with the ng-model and the ng-checked, but I've tried every combination I can think of.

Astonishingly, the angular docs and examples do not seem to address this simple case as far as I have found.