command parameter getting null in treeview contextmenu

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WPF

Hi,

In treeview I want item object on which context menu is open through command parameter. But I am getting null value currently.

Sample,

<treeview.contextmenu>
              <contextmenu>

           <menuitem header="Check">
                        Command="{Binding StartPuttyCommand}"
                        CommandParameter="{Binding RelativeSource={RelativeSource   AncestorType=ContextMenu}, Path=PlacementTarget.SelectedItem}"/&gt;
              </menuitem></contextmenu>
</treeview.contextmenu>

Thanks in Advance
Brinda.