Thank you for your report!
From what I can tell this only happens with pure black (ColorRGBA( 0, 0, 0, alpha )), so you may be able to work around this bug by changing the color slightly.
You can download colors.xml here: http://www.imagemagick.org/script/resources.php. Copying that file into the Draft directory (\path\to\Draft\Windows\64bit etc) seemed to fix the problem for me.
Edit: in the next Draft release, this will appear as a warning (which you can safely ignore) instead of an error.