So it’s especially notable when the Take-Two CEO says he has been shocked by the continued success of Grand Theft Auto (GTA) V. Since the game’s release four years ago, it has sold more than 80 million units—and continues to be a major part of the videogame distributor’s earnings.

In its quarterly earnings, which were released Wednesday afternoon, the company said sales of the game were “substantially exceeding” expectations; GTA V was the single biggest contributor to recurrent consumer spending, which saw sales grow 71% year-over-year (and which made up 58% of the company’s total net sales).

“We had predicted it would peak last year,” Zelnick tells Fortune. “So far, we’ve been happily wrong. … What we’ve found is when we put out new content, people are excited to participate and that affects our recurrent spending positively. That said, four years after the initial release, it’s hard to have expectations. We learn new things about GTA‘s resilience on a daily basis.”

One thing’s for certain: The company’s not planning to underestimate it moving forward. Take-Two says it now expects the game to continue to see net sales growth in fiscal 2018—and once again set records.

Of course, no game can continue to grow forever—but Take-Two has another potential blockbuster in the wings with Red Dead Redemption 2, the sequel to the 2010 hit open world western game, due in the Spring of 2018.

Zelnick hasn’t given any hints about what to expect, but he says the company is just as confident about Red Dead as it has become about GTA.