Anyone tracking the number of businesses Elon Musk is involved with can add one more to the list: merchandise. Specifically, a flamethrower. His tunneling startup, Boring Co., started selling a branded weapon online over the weekend.

Thanks to the billionaire’s promotions on Twitter and Instagram, Boring Co. had taken orders for thousands of the $500 flamethrower by this morning. The total now exceeds 10,000 units, worth $5 million, said a spokesman for the company. Boring Co. plans to sell 20,000 flamethrowers.

Musk embraced the unusual nature of his supervillian-esque side business. He joked on Twitter that the weapons would help in a zombie apocalypse but that he’s not secretly raising an undead army to generate demand. He changed his Twitter bio to “Zombie Defender.”

The danger of selling a fire-emitting device and the lighthearted way Musk is treating the topic raised questions about whether it’s a joke and the legality of selling such a weapon online. The company says it’s serious.

Boring Co. was founded in late 2016 to satisfy Musk’s ambitions to build underground transportation systems. The company began digging a 30-foot-wide test tunnel in Los Angeles a year ago and is talking with officials there and in Culver City, California, about extending the tunnel to West Los Angeles. Last year, Boring Co. obtained a permit to begin digging under a stretch of a Maryland highway as part of a potential hyperloop, a high-speed transportation concept using tubes outlined by Musk. The hyperloop would ferry people between Washington, D.C., and New York City.

The East Coast tunnel would likely be a years-long, multibillion-dollar project requiring approval from multiple jurisdictions, including the U.S. capital. City officials there have met with Musk’s team and say they’re open to ”exploring options that improve mobility for residents and visitors to the nation’s capital,” according to a spokesman for Washington’s Department of Transportation. Boring Co. has said it’s not seeking government funding.

In the meantime, Boring Co is peddling merch. Last year, it sold 50,000 navy blue baseball caps with the company name stamped above the visor at $20 apiece. That generated $1 million. At the time, Musk promised on Twitter that if the company sold 50,000, it would start selling flamethrowers.

Musk has used unconventional techniques to raise capital in the past. For the much-anticipated Model 3 electric car, Tesla Inc. took $1,000 deposits from customers to reserve a spot in line to buy one. Boring Co. is largely financed by Musk himself, according to executives presenting at the Culver City council meeting last week. They said the company didn’t depend on hat sales for revenue.

In addition to flamethrowers, fans can spend an extra $30 on what Boring Co. describes as an overpriced fire extinguisher.

“You can definitely buy one for less elsewhere,” the website explains. “But this one comes with a cool sticker.”