Ribbon Button Doesn't Respond to Change in "IsEnabled"

<Window 
    x:Class="DockingSample.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:sample="clr-namespace:DockingSample"
    xmlns:ribbon="clr-namespace:ActiproSoftware.Windows.Controls.Ribbon.Controls;assembly=ActiproSoftware.Ribbon.Wpf30"
    Title="Window1"
    WindowState="Maximized"
    >
    <Window.Resources>
        <BooleanToVisibilityConverter x:Key="BooleanToVisibilityConverter" />
    </Window.Resources>
    
    <Window.CommandBindings>
        <CommandBinding Command="sample:Window1.MyCommand" CanExecute="CommandBinding_CanExecute" Executed="CommandBinding_Executed" />
    </Window.CommandBindings>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="*" />
            <RowDefinition Height="*" />
            <RowDefinition Height="*" />
        </Grid.RowDefinitions>
        
        <Button
            Margin="3"
            Content="Toggle Command State"
            Click="Button_Click"
            />
        <ribbon:Button
            Margin="3"
            Grid.Row="1"
            Label="Copy"
            Command="sample:Window1.MyCommand"
            Visibility="{Binding RelativeSource={RelativeSource Self}, Path=IsEnabled, Converter={StaticResource BooleanToVisibilityConverter}}"
            />
         <Button
            Margin="3"
            Grid.Row="2"
            Content="Copy"
            Command="sample:Window1.MyCommand"
            Visibility="{Binding RelativeSource={RelativeSource Self}, Path=IsEnabled, Converter={StaticResource BooleanToVisibilityConverter}}"
            /> 
    </Grid>
</Window>

using System.Windows;
using System.Windows.Input;

namespace DockingSample
{
    /// <summary>
    /// Interaction logic for Window1.xaml
    /// </summary>
    public partial class Window1
    {
        // Fields
        private bool _allowExecute;

        // Constructors

        public Window1()
        {
            InitializeComponent();
        }

        static Window1()
        {
            MyCommand = new RoutedUICommand("MyCommand", "MyCommand", typeof(Window1));
        }

        // Properties
        
        public static RoutedUICommand MyCommand { get; private set; }

        // Event Handlers

        private void Button_Click(object sender, System.Windows.RoutedEventArgs e)
        {
            _allowExecute = !_allowExecute;  
        }

        private void CommandBinding_CanExecute(object sender, CanExecuteRoutedEventArgs e)
        {
            e.CanExecute = _allowExecute;
        }

        private void CommandBinding_Executed(object sender, ExecutedRoutedEventArgs e)
        {
            MessageBox.Show("Command Executed");
        }
    }
}