Shader Mixer Previews

bailaowai

I'm trying to model a couple of shaders in Daz to match some cycles shaders that I'm familiar with.  For some reason I cannot get the preview functionality to work.  It's always black.  Both the final output preview, and the little individual brick previews, are just black when I click them.  The shader DOES apply properly if I select a surface in the scene and click apply, however to troubleshoot and fine tune I really need to see a few of the intermetate shaders in preview mode.

I've searched all over and there is very sparse documentation on this feature.  The only mention is in one page on daz3d.com that says "when you click preview it will show a default scene".  In youtoube videos (from 6 yrs ago) it seems to show a sphere in front of a simple background.

Anybody else have this problem, and/or know how to make this work?

Thanks, Eric