Why can't I build the starting iOS project?

When I try and build the iOS starting project for a PCL with Xamarin.Forms 1.4.3.6376, I get the following build error
Can not resolve reference: /Users/.../Library/Caches/Xamarin/mtbs/builds/BusyDialog2iOS/9c0285e2042ab1edb93c184cbdf089b5/C:/Program Files (x86)/Reference Assemblies/Microsoft/Framework/Xamarin.iOS/v1.0/Facades/System.Collections.Concurrent.dll BusyDialog2.iOS

Any ideas what I need to do to fix this?