PhotoView 360 2011 SP4.0

I'm attempting to create a contour rendering (lines only) and save the output in PNG format (so that it has a transparent background).  It isn't working at all liked I'd expected.  In fact, it saves what appears to be a black/empty image.  I have tried changing the background (none/gradient/color), and the line thickness/color, all without the desired result. 

Attached are the various outputs I tested.  The work around I had to use was to save the file as a JPG, and then edit in Photoshop, removing the all white background it was rendered on, and then saving that as a PNG.  Strangely, if I render both the solid model AND the contour lines, this outputs the PNG file appropriately (see attachment Blue Overlay.PNG)

On a related note, something that I found very strange - if you are rendering ONLY the contour lines, but still have the "Floor Shadows" option selected, PV360 still runs through all the pre-pass phases, even though they aren't being used for the output at all.  If you uncheck this option the rendering is much, much faster, as the software goes directly to what I call "Snake Mode". 

TL;DR - Why can't I save a contour line rendering as a PNG?