[Pharo-project] FFI error message: Heisenbug

I'm working on GSL+callbacks, and have hit a genuine Heisenbug - the behavior changes depending on when/where I break and/or step over or into code. Callbacks are getting hit successfully(!!!) but there is one function that crashes, unless I step over the call, in which case I get an error. In particular, if I step far enough into things to see the call, calling gsl_multifit_fdfsolver_set() is raising an error (vs. crashing if I don't look<g>) that says 'No module to load address from'. Anybody know what that means? I'm using the CogVM on Ubuntu Lucid. Bill