In the Harry Potter universe, wizards teleport through fireplaces, people hold conversations with paintings, and characters fly on brooms in a sport won by capturing a winged golden orb. Yet, apparently, it’s a stretch too far for some fans, who are responding on social media, that a character in an adaptation be played by a black woman.

Reactions to the announcement that Noma Dumezweni would play the now grown-up magic student Hermione Granger in the London play “Harry Potter and the Cursed Child,” based on the novels by J. K. Rowling, were mostly positive. But, the Internet being the Internet, there was more than a smattering of Harry Potter fans opposed to the idea completely.

Ms. Rowling said on Twitter that the only “canon” description of Hermione is “brown eyes, frizzy hair and very clever,” and “white skin was never specified.” That Ms. Rowling should be the ultimate arbiter of her own intentions, as Marshall McLuhan was of his in a ticket line in Woody Allen’s “Annie Hall,” is hard to dispute.

But it’s also a little beside the point. Whether Ms. Rowling had described Hermione as white, black, green or none of the above, casting an adaptation of a work is not the same as illustrating it.