WP8 cannot resolve assembly

Hello, I have an existing, long running Forms project for Android and iOS. Now I'va added WP support. But if I try to compile it, I get the following error :

Type universe cannot resolve assembly: Xamarin.Forms.Platform.WP8, Version=1.5.0.0, Culture=neutral, PublicKeyToken=null.
and
Cannot resolve Assembly or Windows Metadata file 'Xamarin.Forms.Platform.WP8.dll'

I have no idea whats going wrong. I use the latest versions for all components.

BR Axel