The end face had 17 edges. I think that qualifies as an ngon.

Ah. :) But, that "depends", really. Seventeen edges? How in the world did that happen? Was this a boolean-generated object? A .ai generated object? Or... user error? :D Anyway, glad you sorted it out. As it stood, as shown in the somewhat "hidden wireframe" (kind of a wire at any rate) render, if it was constructed like that, then a few extra edges and, perhaps, chamfers long the long-side, would probably take care of the shading issue.