PCL for iOS, Android and Windows Phone 8

Hi,

I am trying to create a PCL (Portable Class Library) in VS2012 that I plan to use for my iOS, Android and Windows (Store and Phone) app. However, when I select the option for Windows Phone 8 along with Xamarin.Android and Xamarin.iOS I get the following message.

There is no available functionality that is portable between the frameworks you have selected.

Selecting the option for Windows Store App instead of the Phone SDK, doesn't show the above message.

Can someone please explain the message? Because I was under the impression that my PCL would work for all 4 OS (viz., Android, iOS, Windows Store and Windows Phone)

Regards, DipteeR