Treeview ContextMenu Command Binding

I am having problems binding to a ContextMenu attached to treeview items.  The sample code has two treeview's in a stackpanel.  The first treeview has one item with a header of "Group 1"  the seond treeveiw has one parent node with the header "Group 2" and three subnodes.  I want to attach a contextmenu to the header node "Group 2" that has an "Add Item" context menu and I want to add a context menu to the three subnodes of "Group 2" to delete the item.  In the code shown below the AddItemCommand is never executed but the DeleteItemCommand is executed.  However, the later has a null object.  I would like the DeleteItemCommand to be passed either the subnode item index (or item) that was selected prior to the contextmenu being displayed.  The UserControl DataContext is set to the InstallationViewModel during construction.

<UserControl x:Class="Trans.Views.InstallationView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:prism="http://www.codeplex.com/prism"
             xmlns:i="clr-namespace:Microsoft.Practices.Prism.Interactivity;assembly=Microsoft.Practices.Prism.Interactivity"
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300" BorderBrush="Transparent" BorderThickness="0"><StackPanel Background="White"><StackPanel.Resources><ContextMenu x:Key="ParentContextMenu"><MenuItem Header="Add..." Command="{Binding Path=AddItemCommand}" /></ContextMenu><Style x:Key="TheStyle" TargetType="{x:Type TreeViewItem}"><Setter Property="IsExpanded" Value="True" /><Style.Resources><SolidColorBrush x:Key="{x:Static SystemColors.HighlightBrushKey}" Color="DodgerBlue"/><SolidColorBrush x:Key="{x:Static SystemColors.ControlBrushKey}" Color="Transparent"/><SolidColorBrush x:Key="{x:Static SystemColors.InactiveSelectionHighlightBrushKey}" Color="Transparent"/></Style.Resources></Style></StackPanel.Resources><TreeView ItemContainerStyle="{StaticResource TheStyle}" BorderBrush="Transparent" HorizontalAlignment="Stretch"  ><TreeViewItem Header="Group 1" ></TreeViewItem></TreeView><TreeView ItemContainerStyle="{StaticResource TheStyle}" Background="White" HorizontalAlignment="Stretch" BorderBrush="Transparent" ><TreeViewItem Header="Group 2" ItemsSource="{Binding Path=Items}" IsExpanded="True" Focusable="true"><TreeViewItem.HeaderTemplate><DataTemplate><TextBlock Text="{Binding}" ContextMenu="{StaticResource ParentContextMenu}"/></DataTemplate></TreeViewItem.HeaderTemplate><TreeViewItem.ContextMenu><ContextMenu><MenuItem Header="Delete" Command="{Binding Path=DeleteItemCommand}" CommandParameter="{Binding Path=SelectedIndex}"/></ContextMenu></TreeViewItem.ContextMenu></TreeViewItem></TreeView></StackPanel></UserControl>

    [Export]
    public class InstallationViewModel
    {
        public ObservableCollection<string> Items { get; set; }

        public DelegateCommand AddItemCommand { get; private set; }

        public DelegateCommand<Object> DeleteItemCommand { get; private set; }

        public InstallationViewModel()
        {
            Items = new ObservableCollection<string>();
            Items.Add("Item 1");
            Items.Add("Item 2");
            Items.Add("Item 3");

            AddItemCommand = new DelegateCommand(OnAddItem);
            DeleteItemCommand = new DelegateCommand<Object>(OnDeleteItem);
        }

        private void OnAddItem()
        {
            var i = 1;
        }

        private void OnDeleteItem(Object obj)
        {
            var i = 1;
        }

    }