If you haven’t seen the ads for it, fast food giant Burger King is dishing up a variation of its Whopper in time for Halloween that celebrates one of that holiday’s two signature colors. Specifically, the seeded bun that sandwiches the normal quarter-pound burger plus all the usual fixins’ has been dyed black. Judging from a candid snapshot, the limited-edition Whopper isn’t all that appetizing.

But it’s not just the bread that undergoes a color shift. As a number of commenters on Twitter warn, the chemicals used to dye the bun black bring about other, probably unanticipated changes as well. According to consumer website The Impulsive Buy, the dyes in the sandwich may turn your poop green. Among the ingredients listed are FD&C Red #40, FD&C Blue #1, raisin juice concentrate, and FD&C Yellow #6, any or all of which could be the culprit.

Admittedly, BK’s commercial photographers have managed to make the specialty item look somewhat more palatable.

But not even the most gifted graphic artist can make the sandwich look more appetizing in its post-digestion stage.