Gallery layout breaks when using an Imagecache Define Canvas action

This is an odd one.

When modifying imagecache presets I tried to add an "Define Canvas" action to my preset and ended up rendering a gallery with thumbs that layer over top of each other! The grid completely breaks. (all my imagecache preset does is scale to 96x96 and add a black background) I can't think of why that would break the layout.

The weird thing is that using other imagecache presets seems to work fine (though I didn't test all) I did a desaturate, and tried changing the scales, etc.

The only solution I found was to add an image file as a background using the action "Underlay (background)" That gives me the exact same thing as before: a 96x96 pixel image with a blackground. But this doesn't break the gallery layout at all!!

( I attached a composite of screenshots in case my explanation doesn't make sense )