Separator casuses CanExecute bug

<Window x:Class="WpfApplication3.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:ribbon="http://schemas.actiprosoftware.com/winfx/xaml/ribbon"
    Title="Window1" WindowState="Maximized">
    <Window.Resources>
        <ribbon:RibbonCommand x:Key="ChangeAlignmentCommand" Label="CommandLabel"/>
    </Window.Resources>
    <Window.CommandBindings>
        <CommandBinding Command="{StaticResource ChangeAlignmentCommand}" CanExecute="CommandBinding_CanExecute"/>
    </Window.CommandBindings>
    <ribbon:Ribbon>
        <ribbon:Tab>
            <ribbon:Group>
                <ribbon:PopupButton Label="{Binding RelativeSource={RelativeSource Self}, Path=IsEnabled}">
                    <ribbon:Menu>
                        <ribbon:Button Command="{StaticResource ChangeAlignmentCommand}" CommandParameter="Left" Label="Left"/>
                        <!--<ribbon:Separator/>-->
                        <ribbon:Button Command="{StaticResource ChangeAlignmentCommand}" CommandParameter="Top" Label="Top"/>
                    </ribbon:Menu>
                </ribbon:PopupButton>
            </ribbon:Group>
        </ribbon:Tab>
    </ribbon:Ribbon>
</Window>

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