Katy Perry on music tie-up with Swift : If she says sorry, sure!

Katy Perry is “open” to collaborating with Taylor Swift on a new song, but on one condition.

In a response to a fan’s curious question about whether she can collaborate with Swift on a new chart-topping single, the “Rise” singer responded by tweeting, “if she says sorry, sure!”

@HerWonkEye @katycatsophia @camilasviews if she says sorry, sure!

— KATY PERRY (@katyperry) September 10, 2016

There has been bad blood between the two singers for years—to think that, according to E! Online, the two singers were once close buddies who attended the Grammy Awards together and praised each other all the time. In fact, Perry, 31, passionately duetted with Swift, 26, in her concert at Los Angeles back in 2010.

Sadly, their friendship turned topsy-turvy when three of Swift’s backup dancers left her 1989 world tour in 2013 to join Perry’s tour, leaving Swift in a furious mode. In a 2014  interview with Rolling Stone, the “Bad Blood” singer lamented about a fellow artist who “robbed” several members of her staff. “It had to do with business. She basically tried to sabotage an entire arena tour. She tried to hire a bunch of people out from under me,” Swift said.

“For years, I was never sure if we were friends or not,” Swift further exclaimed. “She would come up to me at awards shows and say something and walk away, and I would think, ‘Are we friends, or did she just give me the harshest insult of my life?'”

Sensing Swift is throwing some shade at her, Perry posted a cryptic tweet on Sept. 9, in which fans speculated was a “swift” lash against Swift. “Watch out for Regina George in Sheep’s clothing…” she said.  Gianna Francesca Catolico

Watch out for the Regina George in sheep's clothing…

— KATY PERRY (@katyperry) September 9, 2014