Sweep 1 rail problem

Hi all ,
I have a problem with sweep 1 rail.
I created curves in Rhino ( Closed and Planar ) and I can’t use sweep 1 rail .
Please look at my file and help me understand why it won’t accept it .
Thank you !

Sweep 1 rail problem.gh (10.0 KB)

This could be a way, but not always successful…

Sweep 1 rail problem_re.gh (17.2 KB)

Thank You !
That works , but like you said , not all the time and I wonder why it doesn’t when i move the slider up and down .
I’m just trying to understand the reason behind the Fail .