Disallow unchecking of all checkboxes

I'm using AngularJS directive for list of checkboxes checklist-model but I need to keep at least one check box checked at all times. I've changed the checklist-model.js by adding an if:

watch if(current.length > 1)

before removing a check:

scope.$watch(attrs.ngModel, function(newValue, oldValue) {
  if (newValue === oldValue) { 
    return;
  } 
  var current = getter(scope.$parent);
  if (angular.isFunction(setter)) {
    if (newValue === true) {
      setter(scope.$parent, add(current, value, comparator));
    } else {
        if(current.length > 1)
        {
            setter(scope.$parent, remove(current, value, comparator));
        }
    }
  }

  if (checklistChange) {
    checklistChange(scope);
  }
});

The problem is that the UI has already changed to unchecked while the model did not change.

Anyone has a suggestion on how to change the checkbox back to "checked" state, or even better to catch it before it changed to checked?

JSfiddle here, try to uncheck all and see that when the check boxes are all unchecked (what I'm trying to prevent) the model stays not empty (which is good but not enough)