Class Library (Portable) UWP + WPF

I created a simple Portable Class Library project that is being used in two WPF e UWP projects.

Both projects work normally, however, the UWP project while compiling displays the following error message:

AlthoughtheprojectworksnormallyIwasintriguedbythismessage,wouldanyoneknowthereason?

project.json

{"supports": {
    "net46.app": {},
    "uwp.10.0.app": {}
  },
  "dependencies": {
    "Microsoft.NETCore.Portable.Compatibility": "1.0.1",
    "NETStandard.Library": "1.6.0"
  },
  "frameworks": {
    ".NETStandard1.3": {}
  }
}