C# - WPF Dependency Property Not Working?

I have a custom Dependency Property defined like so:

public static readonly DependencyProperty MyDependencyProperty =
DependencyProperty.Register(
"MyCustomProperty", typeof(string), typeof(MyClass));[code]....

Now I try set that property in XAML

<controls:TargetCatalogControl MyCustomProperty="Boo" />

But the setter in DependencyObject never gets hit! Although it does when I change the property to be a regular property and not a Dep Prop