RibbonComboBox SelectionChanged not firing

I'm developing an app in .NET 4.5.1 WPF/C#. I have added the Microsoft RibbonBar to the main window. On it, I have a RibbonComboBox which I populate at runtime with labels. Wen the user selects an item from the RibbonComboBox, however, it is not firing the SelectionChanged event.

Here is the code that populates the RibbonComboBox:

private void PopulateRibbonComboBox()
{
    Label label;    
    label = new Label();
    label.Content = "a";
    ribbonComboBox.Items.Add(label);

    label = new Label();
    label.Content = "b";
    ribbonComboBox.Items.Add(label);

    label = new Label();
    label.Content = "c";
    ribbonComboBox.Items.Add(label);
}

Here is the XAML for the RibbonBar:

<Ribbon><RibbonTab Header="Tab"><RibbonGroup Header="Group"><RibbonComboBox x:Name="ribbonComboBox" Label="Select an item:"><RibbonGallery Name="ribbonGallery" SelectionChanged="ribbonGallery_SelectionChanged"></RibbonGallery></RibbonComboBox></RibbonGroup></RibbonTab></Ribbon>

And finally, the C# code that handles the SelectionChanged event:

private void mapTypesRibbonGallery_SelectionChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
{
    RibbonGallery source = e.OriginalSource as RibbonGallery;
    if (source == null) return;
    Label selectedLabel = (Label)source;
}

However, when I run the program and select an item from the RibbonComboBox, the SelectionChanged event doesn't fire. Any ideas where I'm going wrong? I'm quite new to WPF/XAML and very new to the RibbonBar.

Thank you...

Fabricio Rodriguez Pretoria, South Africa