My eye is turning black when adding the transmission on the BSDF node I'm trying to make an eye and while adding the outer later that makes it shine it's turning it dark instead. 
It's supposed to look like this:

But ends up looking like this:


Does anyone know what's up?
Here's my blend file.
He's in Eevee, maybe that setup works fine in Eevee, but in Cycles I think you need at least to mix your Principled BSDF (with 1.00 Transmission) with a Transparent BSDF node. There is a lot of solutions (like use Fresnel in the Mix factor, use Glossy, etc) but it will always be a mix with Transparent BSDF in my opinion.