Cannot install Xamarin.Forms 1.1.1: "A numeric comparison was attempted"

I'm trying to update to Forms 1.1.1 but installation via Nuget fails in VS:

'Xamarin.Forms 1.1.1.6206' already installed.
Adding 'Xamarin.Forms 1.1.1.6206' to 03 CashTrackerX.iOS.
A numeric comparison was attempted on "$(TargetPlatformVersion)" that evaluates to "" instead of a number, in condition "'$(TargetPlatformVersion)' > '8.0'".  C:\Program Files (x86)\MSBuild\12.0\bin\Microsoft.Common.CurrentVersion.targets

Any ideas how to fix this?