Bug: Border and Clear space contrast: NaN percent

I have a border - solid black composite path and a clear space - solid white composite path. All aligned to the contour. I see "Contrast in your design is NaN percent".

Here's a simple AI file with just those elements, so it's as simple as possible to diagnose the issue: https://www.dropbox.com/s/1llm7tuhu9h2arj/bug-repro.ai?dl=1 (pretty odd - the forum software here doesn't support AI file attachments....)

Perhaps you could provide an alternative script with "override" feature, to work around this bug, so I can export this mark, which seems clearly OK to me?

Please help!