DatePicker crashing my application ( setBinding)

Hello, I got a DatePicker on my Edit/Create item page and the page is crashing everytime I open it.

I've got a model that has a Date on string type and I want to show it as a DatePicker. Also when I'm editing an object I want to be able to edit it by picking a DateTime on my DatePicker and convert it to string .

I've made already the Converter Class:

public class DateTimeToStringConverter : IValueConverter
{
    public object Convert (object value, System.Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        var s = value as string;

        if (s == null) {
            throw new NotImplementedException ();
        }
        else {
            DateTime myDate = DateTime.ParseExact (s, "dd-MM-yyyy",System.Globalization.CultureInfo.InvariantCulture);
            return myDate;
        }

    }

    public object ConvertBack (object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        var s = (DateTime)value;

        if (s == null) {
            throw new NotImplementedException ();
        }
        else {
            string ret = s.Date.ToString ();
            return ret;
        }
    }
}

And this is the way I set the binding to the button:

        var dateLabel = new Label { Text = "Date" };
        var dateEntry = new DatePicker ();
        dateEntry.SetBinding (Entry.TextProperty, new Binding ("Date", BindingMode.TwoWay, new DateTimeToStringConverter (), null));

For some reason its crashing my application when I try to open that page. Anyone could give me an help ?
Thanks in advance