weird black 'outlines' with cycles glossy shader

I know this is not a realistic material, but I wanted to isolate the problem…

Any idea what’s causing it, and how I can solve it?

The image on the right is the normal map used on the right-hand ‘sphere’. It was baked in Blender internal, but I don’t see how that would make a difference.


https://blenderartists.org/forum/showthread.php?401501-Usefull-node-setups-that-solves-problems-(-Cycles-)&p=3069727&viewfull=1#post3069727