Katy Perry revealed that she made a big mistake ― huge ― in passing up an early opportunity to work with fellow entertainer Billie Eilish.

The “Teenage Dream” singer spoke about the missed collaboration in a now-viral TikTok posted by the Los Angeles-based radio station 102.7 KIIS-FM on Saturday.

Perry recalled an intro made by someone who told her, “Hey ― check out this new artist. I’d really like us to work with her.”

“It was a song called ‘Ocean Eyes’ and it was just a blond girl,” the singer said, dropping the name of Eilish’s overnight smash that debuted in 2016.

“I was like, ‘Meh, boring,’” Perry said. “Big mistake. Huge mistake.”

“Don’t let this hit the internet,” the singer said with a smile.

The song ended up becoming a smash hit for Eilish and her brother, Finneas, who together wrote and produced the single.

But it was never their intention for the song to become popular, as the singer revealed it was originally written for her brother’s band and then performed by Eilish for her dance teacher.

“One of my teachers asked if I would either write a song or have my brother write a song to choreograph a dance to,” Eilish said in an interview with Teen Vogue in 2017. “I was like, ‘Yes, that’s such a cool thing to do!’”

“We put it on SoundCloud with a free download link next to it so my dance teacher could access it,” she added. “We had no intentions for it, really. But basically overnight a ton of people started hearing it and sharing it.”