Uncaught Error: Assertion Failed: You must use Ember.set() to set

After updating ember-cli to 0.1.5 i am getting a new error.

 var radioList = this.get('radioList').slice(0);
        radioList.forEach(function (item) {
            item.isSelected = false;
        }); 
         this.set('radioList',radioList);

radioList property is being observed. However in the above scenario it is being sliced which is basically creating a new copy of it. How come even then i am getting the below error?

Uncaught Error: Assertion Failed: You must use Ember.set() to set the isSelected property (of [object Object]) to false.