Angularjs ng-model doesn’t work inside ng-if

Here is the fiddle showing the problem. http://jsfiddle.net/Erk4V/1/

It appears if I have an ng-model inside of an ng-if, the model does not work as expected.

I am wondering if this is a bug or if I am misunderstanding the proper usage.

<div ng-app >
    <div ng-controller="main">

        Test A: {{testa}}<br />
        Test B: {{testb}}<br />
        Test C: {{testc}}<br />

        <div>
            testa (without ng-if): <input type="checkbox" ng-model="testa" />
        </div>
        <div ng-if="!testa">
            testb (with ng-if): <input type="checkbox" ng-model="testb" />
        </div>
        <div ng-if="!someothervar">
            testc (with ng-if): <input type="checkbox" ng-model="testc" />
        </div>

    </div>
</div>