[magick-users] A grid appears when resizing. It wasn't invited. (Bug?)

[Seth Price]

Hello magick folks! When I run the following command, a black grid  
appears on the resulting output.

montage -mode Concatenate -filter "Point" -resize 5400 inf_mor_w.png  
inf_mor_e.png 2km.png

I don't think this grid always appears, and I don't know what's  
special about my input images, so  I've put them online here:

http://leopold.sage.wisc.edu/testing/inf_mor_e.png
http://leopold.sage.wisc.edu/testing/inf_mor_w.png

I haven't had a problem until now, and I'm running the latest stable  
version of IM on a Mac. (problem also occur on the previous version  
of IM that I was running from a few months ago, but I don't remember  
which that was.)
Thanks,
Seth