[wlesieutre]

I tried it again today and it worked, so there must have been some other reason it was failing. Maybe one of my lines was out of plane? But then I'm not sure why adding a midpoint on the straight edge fixed it. I must have accidentally fixed some other problem at the same time.

Here's another odd note though, if you have a polyline with a single arc and use properties to set it to closed, it adds a straight edge and you get a D shape. If you use PEDIT CLOSE it adds another arc to close it instead, and it tries to continue tangent at the end, completing a whole circle.