Cannot use PCL in iOS app: Incompatible target framework (.NET Portable Subset) - Xamarin Studio

Hello, in Xamarin Studio I cannot seem to use a trivial PCL.

• Make an empty PCL, set it to Profile104 as it is the only one supporting MonoTouch and Mono Android
• In same solution add a iOS Library project (or iPhone app or whatever)
• Drag the PCL project into the iOS library references.

The reference is red and the error is "Incompatible target framework". I tried switching to other profiles (136 etc) to no avail. The PCL can be referenced without issue within other PCL projects.

I should probably add that I am using XS 4.3 build 52 on the Alpha channel, and that the same PCL will work seamlessly in a Mono project (which targets 4.5).

Any help appreciated.