IconWorkshop and .icns problems

Does anyone have any experience using IconWorkshop? I am trying it out as I do most of my photoshop/Illustrator work on my desktop which is a PC. The problem that I'm having is that the .icns files that it produces do not work... When I view the icns files (on my Mac) in preview the 16x16 through 128x128 sizes are fine but then the 256x256 and 512x512 sizes are messed up.

#496396