ObservableCollection in PCL causing AOT error

In a PCL, I have a subclass of ObservableCollection, which I'm trying to use in a Xamarin.iOS project. This is causing an AOT error when trying to build for iOS devices. Is this a Xamarin bug? I've attached an example project and the build output.

Creating a PCL in Visual Studio allows the use of ObservableCollection by default, but in Xamarin Studio I need to add a reference to System.Windows... why the difference?

I'm using Xamarin.iOS 6.9.7.13 and Mono 3.2.2.

Thanks

Tom