ComboBox with null values in DataTemplate

Question

I have a problem with null values using a ComboBox in a DataTemplate.

First screendump shows two combo boxes with a DataTemplate.

This first selected value is blank. The last selected value is correct.

<Window x:Class="Test.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:Test"
    Title="MainWindow" Height="350" Width="525">
<Window.Resources>
    <DataTemplate DataType="{x:Type local:PropertyValueViewModel}" >
        <ComboBox Margin="9" SelectedValue="{Binding Value}" ItemsSource="{Binding SelectableValues}" DisplayMemberPath="Description" SelectedValuePath="Value" />
    </DataTemplate>
</Window.Resources>
<StackPanel>
    <ContentControl Content="{Binding ValueSelector, UpdateSourceTrigger=PropertyChanged}"></ContentControl>
    <ContentControl Content="{Binding ValueSelector, UpdateSourceTrigger=PropertyChanged}"></ContentControl>
</StackPanel>

If I uses combo boxes without data templates both combo boxes shows correct values.

<Window x:Class="Test.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:Test"
    Title="MainWindow" Height="350" Width="525">
<StackPanel>
    <ComboBox SelectedValue="{Binding ValueSelector.Value}" ItemsSource="{Binding ValueSelector.SelectableValues}" DisplayMemberPath="Description" SelectedValuePath="Value"/>
    <ComboBox SelectedValue="{Binding ValueSelector.Value}" ItemsSource="{Binding ValueSelector.SelectableValues}" DisplayMemberPath="Description" SelectedValuePath="Value"/>
</StackPanel>

Code behind:

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        ValueSelector = new PropertyValueViewModel()
        {
            SelectableValues = new List<SelectableValue>()
            {
                new SelectableValue("NULL", null),
                new SelectableValue("1", 1)
            },
            Value = null
        };

        DataContext = this;

    }

    public static readonly DependencyProperty ValueSelectorProperty = DependencyProperty.Register(
        "ValueSelector", typeof(PropertyValueViewModel), typeof(MainWindow), new PropertyMetadata(default(PropertyValueViewModel)));

    public PropertyValueViewModel ValueSelector
    {
        get { return (PropertyValueViewModel)GetValue(ValueSelectorProperty); }
        set { SetValue(ValueSelectorProperty, value); }
    }
}

/// <summary>
/// My viewModel
/// </summary>
public class PropertyValueViewModel
{
    public object Value { get; set; }
    public object SelectableValues { get; set; }
}

/// <summary>
/// The items in the combobox
/// </summary>
public class SelectableValue
{
    public SelectableValue(string header, object value)
    {
        Value = value;
        Description = header;
    }

    public object Value { get; set; }

    public string Description { get; set; }
}

Why does the ComboBox change behaviour for null values?

I have created a demo sample here: http://1drv.ms/1fAkP1d.