Validation on nested objects not working after changing whole object

Hi, I have a problem with validating nested objects.
I have following view model:

      var ViewModel = function (first) {
           this.person = ko.observable(new Person(first));

           this.changePerson = function () {
               this.person(new Person("changed"));
           };
      };

      var Person = function (first) {
           this.firstName = ko.observable(first).extend({
                required: true
           });
      };

everything works as expected as long as I do not call changePerson function. After calling change if I have element with data-bind="person().firstName" validation messages will not show. Working example (http://jsfiddle.net/MDhUR/). I found a work round to this issue - if I use to bind to firstName everything works as expected (http://jsfiddle.net/seRr5/1/) .

regards.

Reproduce the behavior using the two linked JSFiddles, focusing on changePerson and the person().firstName binding. Compare it with the ko with person workaround; done means validation messages still appear after replacing the whole person object without requiring that workaround.