"Xam.PCL - Settings Plugin" gives error in test project

I have recently added the Xam.Plugins.Settings to my Xamarin forms project and it is working fine - but it is now failing my unit tests. My unit test are agains ta PCL library but it gives the following error

System.AggregateException: One or more errors occurred. ---> System.NotImplementedException: This functionality is not implemented in the portable version of this assembly.  You should reference the Xam.Plugins.Settings NuGet package from your main application project in order to reference the platform-specific implementation.
   at Refractored.Xam.Settings.CrossSettings.get_Current()

Any ideas? Need to get these tests running really.