Discovery and Michael Phelps are defending a recent episode of Shark Week in which the Olympic gold medalist swam against a CGI shark. Some viewers reacted angrily, complaining that it wasn’t a real shark and they felt duped by promos for the episode.

“In Phelps Vs Shark we enlisted world class scientists to take up the challenge of making the world’s greatest swimmer competitive with a Great White,” the network said in a statement to Deadline. “The show took smart science and technology to make the challenge more accessible and fun. All the promotion, interviews and the program itself made clear that the challenge wasn’t a side by side race. During Michael’s pre-show promotion, as well as within the first 2 minutes of Phelps Vs Shark, this message was clear and we are thrilled with the audience and the engagement around the world.”

Phelps made it abundantly clear in pre-show interviews that he wouldn’t be racing a great white side-by-side.

In a Facebook Live session Tuesday, he said he felt the reaction was just an excuse for some people to express anger about something.

“Everybody wants to try to pick on something or say something or complain about something. I had fun racing a shark and seeing those animals up-close and personal,” he said. “If someone actually wants to get in the water and race side-by-side with a great white, go ahead.”

In Phelps Vs Shark, scientists determined the top swimming speed of a great white, and then showed Phelps swimming against a CGI shark based on that data. Phelps lost the race by two seconds. The CGI shark clocked in at 36.1 seconds and Phelps at 38.1.

In a tweet today, Phelps called for a possible rematch.