endless ko async validation

HTML

<script src="https://rawgithub.com/ericmbarnard/Knockout-Validation/master/Src/knockout.validation.js"></script>

JavaScript

ko.validation.rules.asyncValidator = {
    async: true,
    validator: function (val, vm, callback) {
        vm.bar();
            console.log('validating ...');
    }
};
ko.validation.registerExtenders();

vm = {
    foo: ko.observable(),
    bar: ko.observable()
};
vm.foo.extend({asyncValidator: vm});

ko.applyBindings(vm);