"Closed Sweep" grayed out - Sweep 2 Rails

I’m trying to use the sweep 2 rails command to get a closed sweep surface but the closed sweep checkbox is grayed out. How come?

Please post the 3dm file.
I suspect something is going on with one or both rail curves.

Yup, you were right. There was a very tiny imperfection in one of the curves. I split it down the center and mirrored the working side to the left and it gave me the closed sweep.