MSTest does not execute unit tests

When I try to run my unit tests, either in the Visual Studio 2010 IDE or from the commands line (via mstest /testcontainer:my.dll), I get this error:

Failed to queue test run 'kootsookosp@TFSSERVER 2011-06-28 11:00:11': Method not found: 'Void Microsoft.VisualStudio.TestTools.Agent .AgentProcessManager.SetCurrentPlatform(System.Reflection.ProcessorArchitecture, Microsoft.VisualStudio.TestTools.Common.AssemblyClr Version)'.

Most other posts regarding mstest errors suggest an issue with security, but this seems different.

Googling the above error message doesn't appear to show anything much that seems rele开发者_StackOverflowvant.

Is my.dll 64 bits? MSTest does not support that.