Author Rick Riordan is hitting back against social media users who are upset that a Black actor has been cast in the upcoming Percy Jackson and the Olympians TV series, according to the Hollywood Reporter.

On his website, Riordan defended the casting of Leah Sava Jeffries, who is Black, as the character Annabeth Chase, who was described in his books as White.

Addressing critics of the decision to cast the 12-year-old actor, Riordan wrote, “You are judging her appropriateness for this role solely and exclusively on how she looks. She is a Black girl playing someone who was described in the books as white. Friends, that is racism.”

The casting of Jeffries and Aryan Simhadri in the Disney+ series, based on Riordan’s popular children’s book series, was announced earlier this month. Walker Scobell had previously been named as the actor playing the show’s title role.

“The core message of Percy Jackson has always been that difference is strength,” Riordan wrote. “There is power in plurality. The things that distinguish us from one another are often our marks of individual greatness. You should never judge someone by how well they fit your preconceived notions.”

Jeffries’ supporters spoke out for her on social media. Author Dhonielle Clayton tweeted, “Thank you for standing up for Black girls, @rickriordan. It’s been heartbreaking to see this talented actress get treated like this. I’ve been wrecked watching from the sidelines. She is extraordinary. I can’t wait to see her as Annabeth!”

Michael Schaub, a journalist and regular contributor to NPR, lives near Austin, Texas.