System.TypeLoadException

I run through some code, call a web service, get some data back, do some processing and am getting a System.TypeLoadException. I'm not quite sure where to even track this down at. any ideas? I'm using 4.8.3 on a Mac. The only component that is not in the standard set is Json.NET.

Could not load type 'System.Dynamic.IDynamicMetaObjectProvider' from assembly 'System.Dynamic.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'