Ah, solved it (I think). The bootstrapper used to import the Python and C versions of the module leaves sys.modules unpopulated (Does pickle itself may populate it when it finds no module of that name?). I added a setUp method to the unittest class for operator that explicitly sets sys.modules['operator'] to whichever version is being tested at the time so pickle's lookup works as expected. Is that the right solution? New patch uploaded with that change.