Shelling Failure

In the attached file I have a simple shape that I want to shell to about 0.04" inches.

No matter what I do, when I try to remove the surface shown in red, the shelling fails. Why is this and how can I correct?

Probem Shell.3dm (250.2 KB)

something is funky with this surface:

I deleted it- ran and edgesrf (though that was finicky as well) then joined everything back up and the shell seemed to work fine.

noProbem Shell.3dm (154.0 KB)

I wonder why. It’s just a sweep2. The problem it that I need the surface to exactly match one of the sweep frames in order to mate with another part.

use solid offset instead of shell for this type of part. shell is for closed objects- this is a bad candidate for shell-