TypeMock failure building solution with no mocks

I have a NAnt test script that runs one set of tests, then builds a test utility (using NAnt's msbuild task) and runs a few more. The only usage of TypeMock is in the first batch of tests.

The first set of tests runs fine, including the one that uses TypeMock (a simple dynamic mock). So I don't think it's my TypeMock configuration per se. But when it comes time to build the test utility, I get a failure from the msbuild task, 'Could not load file or assembly, 'TypeMock, version 0.0.0.0, ...'. This task runs fine if I don't run the first batch of tests. The test utility has its own solution and project file, with no TypeMock usage. (Though I don't think that matters, as this seems to be happening upon starting msbuild, before it even opens the solution file.)

Any idea of what's going on?

Thanks,
Gary