Robert Kirkman has a lot to be proud of. He has a hit comic book series that has entranced fans as well as a hit television show based on his work, and most recently two different video games inspired by the world he created. However, he has even more to brag about as his 100th issue of The Walking Dead comic book series has become the best-selling comic of the 21st century according to Image Comics.

The issue has already sold out of its initial order of 383,612 copies.

Part of the reason for the higher-than-usual sales could be, as the New York Times pointed out, that fans and collectors flock toward milestone issues, such as number 100 in a series, as well as issues with multiple covers. The 100th issue released a total of nine different covers, which fans flocked to collect.

Another big draw was likely the expectation that something particularly big might happen in the storyline. (It did, but we won’t spoil it for you here.)

The comic book series has also had other awards including receiving the 2010 Eisner Award for Best Continuing Series at San Diego Comic-Con International.

The massively popular AMC television series probably didn’t hurt sales either. Season three of the series is set to begin on Oct. 14.