ColorEditBox does not transmit Value into Binding

<Window x:Class="WpfApplication1.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:editors="http://schemas.actiprosoftware.com/winfx/xaml/editors"
        SizeToContent="WidthAndHeight">

    <UniformGrid Columns="2">
        <Label Content="UpdateSourceTrigger&#13;Default (c1):" />
        <editors:ColorEditBox Value="{Binding BindableColor}" x:Name="c1" />

        <Label Content="UpdateSourceTrigger&#13;PropertyChanged (c2):" />
        <editors:ColorEditBox Value="{Binding BindableColor, UpdateSourceTrigger=PropertyChanged}" x:Name="c2" />

        <Label Content="value of control c1:" />
        <TextBlock Text="{Binding Value, ElementName=c1}" />

        <Label Content="value of control c2:" />
        <TextBlock Text="{Binding Value, ElementName=c2}" />

        <Label Content="value of binding:" />
        <TextBlock Text="{Binding BindableColor}" />

        <Label Content="space for focus:" />
        <TextBox />
    </UniformGrid>
</Window>

namespace WpfApplication1
{
    using System.ComponentModel;
    using System.Windows;
    using System.Windows.Media;

    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window, INotifyPropertyChanged
    {
        private Color? bindableColor;

        public MainWindow()
        {
            this.InitializeComponent();
            this.DataContext = this;
        }

        public event PropertyChangedEventHandler PropertyChanged;

        public Color? BindableColor
        {
            get { return this.bindableColor; }
            set
            {
                this.bindableColor = value;
                var handler = this.PropertyChanged;
                if ( handler != null )
                    handler(this, new PropertyChangedEventArgs("BindableColor"));
            }
        }
    }
}