Invalid Directory when using Typemock (TMockRunner)

Hello.  I am new to TeamCity and am trying to set up a build based on my Nant build scripts.  Since I am using TypeMock to mock objects for my test I have been streaming everything through TMockrunner using commands like:

tmockrunner -target 3.0 nant test-build /D:test.category=67559

To run this in TeamCity I have just set up a BAT file runner and just execute the commands that I would normally execute.  The problem I am seeing is that when my unit tests are executed (i.e. when TypeMock is mocking), I get strange errors - generally on an unreleated nant task like al or csc, these will typically throw with an Invalid Directory exception.  When I don't execute the unit tests (but stream everything through tmockrunner anyway), the build is successful.  Note, the Nunit task is not the task that throws.

Has anyone executed unit tests with TeamCity and TypeMock?  If so, what strategy have you used to start TypeMock?

thanks in advance.

dennis.