Input Radio with default value

Using Asp.Net MVC 5 with AngularJS I want a input radio to be checked by default.

According to the link:

How to select a radio button by default?

Assign an initial value to radio button as checked

Just use this:

<label class="btn btn-default">
    <input type="radio" ng-model="ViewBag.Imposto.calcular" value="false">
    Não
</label>
<label class="btn btn-default">
    <input type="radio" ng-model="ViewBag.Imposto.calcular" value="true" checked="checked">
    Sim
</label>

But it is not working, how to solve this?

Edited

With can be seen in the image, even using the above code the option is not checked.