It's the message you get when trying to find an icon that doesn't exist on the default units_32.pcx that comes with C3C (only has 349 spaces for icons). If you use a modified units_32.pcx (with more icons on it), and get the program to recognize/find it, you can get access to icons with numbers greater than 349. If you simply overwrite the default units_32.pcx, you probably have to restart the editor to get it to pick up the change.