I'm trying to make a pair of parts, one of which fits inside the other.  One should feature a loft cut, and the other should feature a loft boss, with identical profiles and guide curves.

The cut works fine, but the boss keeps giving me errors about the curve not intersecting the section plane?

Also, why does the boss want to twist around the guide curve (screen images below)?

This is the cut.  This is fine.

This is the boss.  WTF?