PopupButton allways enabled when using binding to MenuItems

<Window x:Class="Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:shared="http://schemas.actiprosoftware.com/winfx/xaml/shared"    
    xmlns:ribbon="http://schemas.actiprosoftware.com/winfx/xaml/ribbon"
    Title="Window1" Height="300" Width="400">
    
    <Window.CommandBindings>
        <CommandBinding Command="Copy" Executed="CopyThis" CanExecute="CopyThis_CanExecute"/>
    </Window.CommandBindings>
    
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="100"/>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>

        <ribbon:Group Label="Group" >
            
            <!--Ok-->
            <ribbon:Button Label="Button"
                           Command="Copy"/>

            <!--Ok-->
            <ribbon:PopupButton Label="PopupButton with fixed menu items" 
                                Command="ApplicationCommands.NotACommand">
                <ribbon:Menu>
                    <ribbon:Button Label="Menu 1" Command="Copy"/>
                    <ribbon:Button Label="Menu 2" Command="Paste"/>
                </ribbon:Menu>
            </ribbon:PopupButton>

            <!--Not Ok, PopupButton allways enabled-->
            <ribbon:PopupButton Label="PopupButton with binding"
                                Command="ApplicationCommands.NotACommand"
                                AutoDisableWhenPopupContentIsDisabled="True"><!--or False-->
                <ribbon:Menu ItemsSource="{Binding MenuItems}"/>
            </ribbon:PopupButton>

        </ribbon:Group>
        
        <TextBlock Grid.Row="1" Text="Type text:"/>
        <TextBox x:Name="MyTextBox" Grid.Row="2"/>

    </Grid>
</Window>

Class Window1 

    Private Sub CopyThis(ByVal sender As System.Object, ByVal e As System.Windows.Input.ExecutedRoutedEventArgs)
        MessageBox.Show("CopyThis command activated")
    End Sub

    Private Sub CopyThis_CanExecute(ByVal sender As System.Object, ByVal e As System.Windows.Input.CanExecuteRoutedEventArgs)
        If MyTextBox Is Nothing Then
            e.CanExecute = False
        Else
            e.CanExecute = (MyTextBox.Text > "")
        End If
    End Sub

    Private Sub Window1_Loaded(ByVal sender As Object, ByVal e As System.Windows.RoutedEventArgs) Handles Me.Loaded
        Me.DataContext = Me
    End Sub

    Public Function GetMenuItems() As MenuItem()
        Dim _menuItems(3) As MenuItem
        For i As Int32 = 0 To 2
            _menuItems(i) = New MenuItem
            _menuItems(i).Header = "Menu from code " & i.ToString
            _menuItems(i).Command = ApplicationCommands.Copy
        Next i
        Return _menuItems
    End Function

    Public ReadOnly Property MenuItems() As MenuItem()
        Get
            Return GetMenuItems()
        End Get
    End Property

End Class