DatePicker fail with "firstDate is undefined" when an out of range date is written in the textfield

When a DatePicker is configured with a range (mindate and maxdate) and the user enter a date out of the range in the textfield and click on the datepicker icon there is an javascript error and the calendar don't pop up.

There is a "firstDate is undefined" in this javascript method (line with widget.cfg.setProperty)
Wicket.DateTime.showCalendar = function(widget, date, datePattern) {
if (date) {
date = Wicket.DateTime.parseDate(datePattern, date);
if (!isNaN(date))

{ widget.select(date); firstDate = widget.getSelectedDates()[0]; widget.cfg.setProperty("pagedate", (firstDate.getMonth() + 1) + "/" + firstDate.getFullYear()); widget.render(); }

}
widget.show();
}

To fix the issue you can test the firstDate value after getSelectedDates call. YUI Calendar return undefined value when the selected date is out of the range.