Failed to resolve assembly Microsoft.CSharp

I'm getting this error when I try to deploy to a phone:

error MT2002: Failed to resolve assembly: 'Microsoft.CSharp, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'

How do I figure out what components / classes are causing this?

I've removed all PCL and external components except Microsoft's HTTP Client libs and supporting BLC components.

All the PCL's point to Win8, WinPhone8, iOS, and Android.

Works fine on all the platforms except iOS.

Alpha channel --

Thanks -- Chad