Hi Brian - that shape has a kind of twisted frame, it is not a flat planar shape, so that's why it is a little different for extruding.

MoI will pick an extrusion direction of straight up in z in this instance (when the input curves are not planar), which in this case doesn't give you a good result. Nothing is actually generated because it would be all messed up. But you can click the "Set dir" button inside of Extrude and then click 2 points to give it an extrusion direction going in the x-axis direction, then that will punch out an extrude, but only the side parts of the extrude will be created, it won't be capped off automatically because that capping only happens with planar shapes.

To build the cap in this case, you can use Edit/Separate to break the curve into pieces. There are 2 pieces on one side, join those together so you have 4 curves, and then with 4 curves around in a ring like this, you can use Construct/Network to surface it. You can also use sweep with 2 profiles and the other 2 as rails.

Then you can make a copy of that cap on the other end of the extrude and use Edit/Join to join them together into a solid if that's what you wanted to make there.

I think I might be able to tune up the automatic extrude direction in the future, for this case of a non-planar curve. Instead of choosing the world z axis direction in this case it seems like it would be better to try and look at the shape and see if it is close to planar in some direction and then use the world axis direction that is closest to that direction instead of always z. But I won't be able to put that in for the v1 release though.

- Michael