Web API Project breaks when Unit Testing

I'm currently trying to unit test for my Web Api project. I currently have 3 projects. MyProjet.Api, MyProject.Shared which is a portable library, and MyProject.Tests. MyProject.Shared is shared between the .Tests and .Api. When I run the .Tests, I'm actually calling the .Shared which then calls the .Api. Yesterday, I was getting the same error as I am today which is..

Method not found: 'Void Newtonsoft.Json.Serialization.DefaultContractResolver.set_IgnoreSerializableAttribute(Boolean)'.

How I fixed this yesterday was by reinstalling web api and Newtonsoft references but this does not solve the issue today. I ran the same tests that passed yesterday, but when I run the tests today, they fail and I refresh the web api page and receive this error again. Any suggestions is greatly appreciated.