A thermal printer has a dye donor supply spool with a body and first and second end portions extending from the body. First and second receptacles are positioned for matingly receiving the first and second end portions of the dye donor supply spool. The supply spool is correctly loaded only when the first and second end portions of the supply spool matingly engage the first and second receptacles, respectively. A mechanical stop prevents incorrect end for end loading of a supply spool that has an off center web wound thereon by engaging the web and preventing the end of the spool from engaging the receptacle.