Zdepth buffer doesn't support volume grids.

[stevesideas]

Is there any way of getting this to work?

[stevesideas]

Never mind. I see you have to change to Volumetric geometry. Seems to take quite a bit longer to render.

[Svetlozar Draganov]

Glad to hear you sorted it out, it indeed might take longer to render but it's the way to extract proper render elements information from the Volume Grids.