Sales of The Boring Company’s flamethrower, which seemed to start as a throwaway line to help the company sell hats, have already topped 7,000 units, according to a tweet from Musk early Monday. At current prices, that means he’s taken in $3.5 million.

The flamethrowers went up for sale Sunday with a pre-order price of $500. (Also for sale? A fire extinguisher with a Boring Company sticker on it for $30.) Fans quickly laid down their cash.

The whole idea of a Boring Company flamethrower started as a joke, when Musk said he’d sell one if the company first managed to sell 50,000 hats. But then, Sunday, he posted a video on Instagram of himself playing with a flamethrower. Sales began soon after and they’re expected to begin shipping this spring.

Musk continues to embrace the absurdity of actually selling a flamethrower, though, noting that they’ll be very handy in the zombie apocalypse, which he said he is not planning, by the way.

When the zombie apocalypse happens, you’ll be glad you bought a flamethrower. Works against hordes of the undead or your money back!

— Elon Musk (@elonmusk) January 28, 2018

You’d need millions of zombies for a so-called “apocalypse” anyway. Where would I even get a factory big enough to make so many!?

— Elon Musk (@elonmusk) January 28, 2018

He also reached out to the crypto-currency obsessed world, noting that each flamethrower is “sentient” and comes with “a free blockchain.”