This is a bug, as defined by Baldurdash (fairly reliable site - they've discovered bugs that the game developers often agree with) and you can download a fix for the item
here.
It's up to you whether you install this or not, but I've used various bug fixes from this site previously and never had any problems with them. Additionally, it seems that this bug has been confirmed as an item definition error by official sources though I don't believe it's made it into any of the patches.