Chamfer problem

 From:  Ditto
Aïe : I have swept a rectangle along a helix and wanted to chamfer just the 4 "long" borders, but not the two faces.

I could have chamfered the rectangle before sweeping, but for the sake of understanding, can someone help me out with the correct way?

Attached file has the solid after the sweep of the rectangle on the left, and my unfortunate chamfer on the right. I don't understand why MoI is doing what it's doing (on the right).