Pipe not working for open curve

I’m running Version 7 SR10 (7.10.21256.17001, 2021-09-13).

I cannot get the Pipe command to do anything on a curved open curve. The below screenshot shows the selected rail (a simple curved open curve) and the start and end radii specified. I am about to press Enter, as there are no more radii:

After pressing Enter, I get this:

No pipe is created and the Pipe command is not in the Undo history. If I do the exact same thing on a straight line segment (again, an open curve) it works. My objective is to get a curved pipe with constant radius whose axis is the selected open curve. Why can’t I get this to work?

The file containing the open curve is here:
Pipe Problem.3dm (25.9 KB)

Hi Mike- there are two stacked control points at the lower end of the curve.


Thanks. It would be nice if Rhino could provide suggestions, hints… something when commands fail.