I created a WCF service library project in my solution, and have service references to this. I use the services from a class library, so I have references from my WPF applic
I was looking over my project and I was having this same issue. It turned out to be different versions of the same DLL on the WCF vs. Web Site.
Web site had a newer version of the DLL and the service was referencing an older version of the DLL. Once they were all in sync all worked well.