Surface.ByLoft Error - No sections have been set

Hi, I want to generate a surrounding polysurface using the dynamo script below. But it shows Warning: Surface.ByLoft operation failed. Unable to loft! : LOFT_SDK_SECTIONS_NOT_SET -- No sections have been set and the results are nulls. Does anyone know how to fix it? Thanks!


model: https://drive.google.com/file/d/1Sewb5oSneF-TzaURAKgCrQskGgbotDTN/view?usp=sharing
dynamo: surface by loft.dyn (19.4 KB)

Solved by adding List.Flattern between List Create and Surface.ByLoft. But I am curious that sometimes it can work without the List.Flattern node. For example, an example by Autodesk (https://knowledge.autodesk.com/support/revit-products/learn-explore/caas/screencast/Main/Details/ef983f41-fdbc-4a2f-b0b3-72b4f3aaf1bd.html) as the picture shows below. The ByLoft works without the flattern node. Does anyone know why? Thanks!
image