boundary not well formed

I have made a corridor which looks well.
As next step I want to producea surface of this corridor.
The surface peroduced is completely false.
The street has the form of a "S".
The surface closes the "S" twice.
What can I do when I get the message "the boundary is not well formed"?
How can I force the surface get the same boundaries as the corridor shows?

This might suggest you select the surface boundary points manually, or use the feature lines from the corridor model as surface boundary.

I as well have had this problem. The way I worked around it was to edit the surface and delete lines outside of the boundary. This works ok, but obviously has limitations. I would like to hear about some other solutions. I have run into this when doing diversion channels. I am fine with deleting a few lines, but I am working on very long diversions and have to delete literally thousands of lines.