Using AngularJS v1.5.0, I am trying to bind a radio input field to a Boolean variable that can be : true, false or null. Using the ng-value directive sets a null value to false.

```
<input type="radio" ng-model="foo.value" ng-value="true"  ng-change="update()">
<input type="radio" ng-model="foo.value" ng-value="false" ng-change="update()">
<input type="radio" ng-model="foo.value" ng-value="null"  ng-change="update()"> 

```

Would you have any idea about how this could be done ?
Thank you !