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!

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 ( as the picture shows below. The ByLoft works without the flattern node. Does anyone know why? Thanks!