TeamCity Could not load file or assembly

I have a number of project in my solution and I have a unit test project that is setup and referenced against on of the library projects in my solution.

E.G.

TestProject.Library TestProject.UnitTests (Referencing TestProject.Library)

The test project is using standard MS tests.

I have 3 build steps in my configuration on TeamCity.

1. Nuget package restore
2. Microsoft Visual Studio 2015 build
3. Visual Studio Tests build step running VSTest 2015

The build step is as default with the Test file names pattern of ***UnitTests.dll.

Everything runs fine apart from the Unit Testing step which always complains that it can't find the dependant library TestProject.Library and errors out. It works fine on any tests that don't reference the other library and everything works when testing locally.

It's not the first time doing Unit Tests in TeamCity but I don't remember having this issue before.

Thanks, Scott