Why is my WCF RIA context throwing FileNotFound exceptions during unit tests?

I'm trying to write unit tests involving my WCF RIA entity proxies and when I try to access an entity's parent or child properties through an association I get a FileNo开发者_开发技巧tFoundException when the WebContext class fails to load System.Xml.

For example, I have two entities defined in my web project in Entity Framework - Course and CourseUnit. A Course has many CourseUnit children. I'm not exposing any RIA contexts or anything to my ViewModels, they are all hidden behind interfaces. I'm using Moq to mock these interfaces and return the standard RIA entity proxies to my unit tests.

This works fine when I'm testing things like

Assert.AreEqual("title", course.Title)

where course is an instance of the RIA proxy. However, if I do something like

Assert.AreEqual(0, course.CourseUnits.Count)

I get the following exception

FileNotFoundException was unhandled by user code Could not load file or assembly 'System.Xml, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e' or one of its dependencies. The system cannot find the file specified.

And similarly if I execute any code in a unit test that involved one of these relationships then the code throws the same exception. Basically if I ever try to access these properties with association relationships from my tests they break.

The exception actually occurs in generated code. It an be found in the file [webProjectName].g.cs in the partial class Course : Entity and happens in the CourseUnits getter.

Any ideas?

Edit - I added a reference to the EntityFramework assembly and the exception has changed to the same thing but System.ComponentModel.DataAnnotations instead of System.Xml. Needless to say I have tried referencing both assemblies in the unit test project, and it didn't work.