WPF MenuItem style parameters not available on menu first open

Question

I am specifying a context menu within the ControlTemplate of a TreeViewItem as follows:

                                    <ContextMenu ItemsSource="{Binding Commands}">
                                        <ContextMenu.ItemContainerStyle>
                                            <Style TargetType="MenuItem">
                                                <Setter Property="Command" Value="{Binding Command}" />
                                                <Setter Property="CommandParameter" Value="{Binding CommandParameter}" />
                                                <Setter Property="Header" Value="{Binding Name}" />
                                                <Setter Property="Icon" Value="{Binding Icon}" />
                                            </Style>
                                        </ContextMenu.ItemContainerStyle>
                                    </ContextMenu>

where Commands is a list of ICommandViewModels objects with the following signature:

public interface ICommandViewModel 
    {
        string Name { get; }
        Image Icon { get; }
        ICommand Command { get; set; }
        object CommandParameter { get; set; }

    }

When the ContextMenu is open, CommandParamter being passed to Command is initially null, which disables Command as specified. If Command.CanExecute always return true, this is not a problem, as Command.Execute eventually gets the correct CommandParameter. In some cases, Command is not allowed to execute if CommandParamter is null, so this becomes a problem.

Anyone with a theory on what is going on here and a possible fix?

TIA.