XamlCompilation causes System.Xml-assembly not found

Hello!
I am using the [XamlCompilation...] option at class level (PCL-Project).
Android and iOS no problems, but i get a "System.IO.FileNotFoundException" at InitializeComponent() - "Could not load file or assembly 'System.XML,Version=4.0.0.0,...." on WInPhone.
Removing the XamlCompilation option i have no problems.

Do you have ever tried the XamlCompilation on WinPhone?
Is there a way to use preprocessor directives in PCL project?

Thanks!
Andreas