C# Controller unit tests failing with bad image format exception, when referenced to service project

C# Controller unit tests failing with the below bad image format exception, when referenced to service project. Both the service and unit test project are being built as AnyCpu. Any suggestions on how to fix this exceptions?

Test method UnitTests.Controller.SomeControllerTests.SampleTest threw exception:

System.BadImageFormatException: Could not load file or assembly SampleService, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies. An attempt was made to load a program with an incorrect format.