America’s favorite fast food nightmare burger now comes with a limited-time bonus side dish in the form of mild digestive problems. To match the spooky black bun of the Halloween Whopper, customers’ excrement has been turning an equally spooky shade of green.

Twitter users and holiday burger-eaters have been reporting the phenomenon all over the internet.

Chris Wilson, who reviewed the freakshow patty over at Maxim, reported that he, too, was stricken by its effects:

The next day I noticed something strange: My poop was a ghoulish shade of green. I assumed the goblin-like pallor of my shit was either due to the A1 Steak Sauce-inspired color of the Whopper bun, or the various other pollutants that course through my system at all times.

Burger King didn’t warn customers about this fun side effect, or perhaps it just didn’t test any of the burgers beforehand. In any case, the company has not issued any explanation. Its nutritional guidelines state that the flavoring and food colorings in the Halloween Whopper, which include Yellow #6, Blue #1, Red #40, and something called “Caramel Color,” are “commonly used in the industry and within the safe and Acceptable Daily Intake approved by the Food and Drug Administration.”

So besides the 710 calories of saturated fat and sodium, a buttload of preservatives, and possibly horse meat, Burger King’s Halloween Whopper is perfectly safe.

Image via Burger King