DependencyProperty on UserControl - PropertyChangedCallback not firing on binding

Hi,

After finishing my AutoCompleteSearchBox UserControl, I now attempt to make it reusable. Therefore, I need to expose some dependency properties such asIList<object> SearchSource and string SearchSourceType.

This is the UserControl:

 public partial class SearchBox : UserControl
    {
public string SearchSourceType
        {
            get
            {
                return (string)GetValue(SearchSourceTypeProperty);
            }
            set
            {
                SetValue(SearchSourceTypeProperty, value);
            }
        }

        public static readonly DependencyProperty SearchSourceTypeProperty = DependencyProperty.Register("SearchSourceType", typeof(string), typeof(SearchBox), new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.AffectsRender, new PropertyChangedCallback(OnPropertyChanged)));

And this is how I use it in another window:

<sb:SearchBox SearchSourceType="{Binding SearchSourceType}"/>

The problem is, OnPropertyChanged never gets called. When I replace the binding with a simple string, it is called just fine. I tried binding it to a different element that I can see to make sure the binding is fine.

Something that might be important to note...the binding is done from a UserControl that is bound to his ViewModel by DataTemplate.

Any help would be apreaciated!