Node previews not displaying in v1.38 Unity 5.6.3p1

There seems to be an issue with the node preview displays. No matter what parameters are fed into them they always appear as a flat white circles. I've attached images of a simple custom lighting setup and the custom lighting shader provided with the asset package. This is version 1.38 within Unity 5.6.3p1 Personal (64-bit) on Windows 7 (x64).