V5 multi-axis machining

Attempting to generate a multi-axis curve maching path
in V5 ... mill a slot in a concave surface. When I attempt
to put in a negative part offset (get the cutter to violate
the surface) I receive a "nothing to mill" message and a
failure to generate the path. I see this message referenced
in the help section under collision checking, but nothing
tells me how to alleviate the problem.
Thanks

Author: Jon Walp