WPF Listbox and Select All

I want to create a simple ListBox and have SelectAll as a context menu item. However it seems that ListBox has some sort of inbuilt handling for SelectAll that I can't get working, but is interfering with my attempt to implement SelectAll.

My entire XAML is this:

<Window x:Class="WpfApplication1.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window1" Height="300" Width="300">
    <Window.CommandBindings>
        <CommandBinding Command="ApplicationCommands.SelectAll"
                        Executed="SelectAllExecuted" />
    </Window.CommandBindings>
    <DockPanel>
        <CheckBox DockPanel.Dock="Top">My Checkbox</CheckBox>
        <ListBox Name="listBox" SelectionMode="Multiple">
            <ListBox.ContextMenu>
                <ContextMenu>
                    <MenuItem Command="ApplicationCommands.SelectAll" />
                </ContextMenu>
            </ListBox.ContextMenu>
        </ListBox>                
    </DockPanel>
</Window>

SelectAllExecuted is simply this:

private void SelectAllExecuted(object sender, ExecutedRoutedEventArgs e)
{
    listBox.SelectAll();
}

Control+A works if the listbox isn't in focus. The context menu item works correctly. But Control+A refuses to work if the listbox is focused.

I feel like I'm fighting against the listbox, but I shouldn't need to.

Edit: It seems my entire problems are with the Multiple SelectionMode. If I set it to Extended then everything works, however I don't want it in extended mode.