Black in glass

[garf 19]

We're looking down on a wineglass with liquid in it.

Anyone know why I'm getting this thick black edge along the top edge of the wine glass? (Left image is refraction pass, right image is beauty pass)

It occurs when I drop raytracing bias down to a small value - say 0.0001 - I need this to be this small in order for my bubbles to render correctly.

The glass shader is a principled shader, ior 1.5, transparency 1, fake caustics on, PBR. 

[garf 19]

so it's definitely the principled shader. If i swap in a classic shader then it goes away.

So it appears to be the principled shader refraction that's causing the issue.

Not yet found a solution though.........

[garf 19]

so after buggering about for far to long on this - it's simply the reflection limit on the mantra rop that needs to be some huge number in order to fix this.

[Midasssilver 13]

There are a few things that could be causing the issue. If you don't have enough things to reflect, like a sky or HDR, that can cause it. Black in your refractions can also come from not having a high enough trace depth.  Changing the bias changes the way refractions are computed, but it wouldn't remove or the black or fix the problem, it is probably just distorting it to angle small enough where you can't see it.