viewModel and bindingContext are undefined in my bindingHandlers

Question

I have the following bindingHandlers:

ko.bindingHandlers.dateRW = {
    init: function (element, valueAccessor, allBindings, viewModel, bindingContext) {
         ko.utils.registerEventHandler(element, "change", function () {
            var value = $(element).val();
            var dateFormatted = moment.utc(value, "DD/MM/YYYY");
            if (dateFormatted !== null && dateFormatted.isValid())
                observable(dateFormatted.toDate());
            else {
                observable(null);
                observable.notifySubscribers(null);
            }
        });
    },
    update: function (element, valueAccessor) {
        var value = ko.utils.unwrapObservable(valueAccessor());
        var date = (typeof value !== 'undefined') ? moment.utc(value) : null;
        var dateFormatted = (date !== null) ? date.format('DD/MM/YYYY') : '';
        $(element).val(dateFormatted);
    }
};

In my view:

<div data-bind="with: transport()" 
    <input type="text" data-bind="dateRW: startDate">
</div>

In the init part you have element, valueAccessor, allBindings, viewModel, bindingContext.

I don't know why but viewModel and bindingContext are undefined.

Any idea?

Thanks.

---

UPDATE

I discovered that if I comment one line in my code:

ko.validation.makeBindingHandlerValidatable('dateRW');

Then I can access viewModel in my init function. It seems to be the problem. Furthermore, current version of knockout.Validation is 1.01 and does not seems to be compatible with what I need (viewModel extra property).

Hope someone can help me on this.

Thanks.