Help. Sweep1 surface disappearing

Sweep 1 created with Record History turned on. When I select a control point on the rail curve, then either move or nudge the point, my surface disappears. Any ideas?

Surface Vanish.3dm (122.2 KB)

Here it does not disappear, it goes the other way - I guess I’d split the rail curve where the section curve sits so it is not ambiguous which direction the sweep should go.

-Pascal

Depends on how much one moves the control point. Large amounts it doesn’t. Smaller amounts it vanishes.

sweep.mp4 (399.1 KB)