在WPF MVVM应用程序中，MultiBindingConvertor无法在网格上工作

<telerik:GridViewComboBoxColumn 
    Header=\"Selection\" 
    DataMemberBinding=\"{Binding HandHeldDifference.GRSSelection}\" 
    ItemsSource=\"{Binding Path=SelectionOptions}\">
    <telerik:GridViewComboBoxColumn.IsReadOnly>
        <MultiBinding Converter=\"{StaticResource MultiBoolConv}\" 
            ConverterParameter=\"True\">
            <Binding 
                RelativeSource=\"{RelativeSource FindAncestor, 
                    AncestorType={x:Type StackPanel}}\" 
                Path=\"DataContext.IsEditable\" />
            <Binding Path=\"IsManualChange\" />
        </MultiBinding>
    </telerik:GridViewComboBoxColumn.IsReadOnly>
</telerik:GridViewComboBoxColumn>

public object Convert(object[] values, 
                        Type targetType, 
                        object parameter, 
                        CultureInfo culture)
{
    var defaultReturn = false;
    if (parameter != null)
    {
        bool.TryParse(parameter.ToString(), out defaultReturn);
    }

    if (values == null) return defaultReturn;
    if (values.Length < 2) return defaultReturn;
    if (values[0] is bool && values[1] is bool)
    {
        return ((bool) values[0]) && ((bool) values[1]);
    }
    return defaultReturn;
}