Using DistanceToNearestSurface in RVTs


I’m trying to create an RVT that will show the location of surfaces near the capturing mesh. Right now the RVT works and functions as I’d expect (the lower plane has a capture volume around it and the upper plane is using the RVT in its material.)

But if I modify the material being captured (the lower plane) to plug DistanceToNearestSurface into it, the output (the upper plane) is just solid black.

Is there something that would prevent this from working, or do I have the wrong idea in my head of how an RVT works?