Before President Trump made his way onto the stage during a campaign rally at the Target Center in Minneapolis on Thursday, thousands of supporters in Make America Great Again gear bobbed along to the familiar guitar licks of hometown star Prince’s “Purple Rain.”

The singer’s estate, though, was less thrilled to hear the track.

In fact, in a tweet shortly after the event, the late artist’s Twitter account said the president’s attorneys had promised to stop playing his music at any Trump events — and attached a letter as proof.

“President Trump played Prince’s ‘Purple Rain’ tonight at a campaign event in Minneapolis despite confirming a year ago that the campaign would not use Prince’s music,” the tweet said. “The Prince Estate will never give permission to President Trump to use Prince’s songs.”

Prince’s estate is among a lengthy list of recording artists who have asked Trump to stop using their music, including Neil Young, Pharrell Williams and the Rolling Stones. More recently, Rihanna demanded that the president cease playing her hit “Don’t Stop the Music” at what she called his “tragic rallies.”

The estate representing Prince, who died in 2016 at age 57, first joined that angry chorus last October after Trump added “Purple Rain” to his regular playlist at his rallies, the Star Tribune reported.

Trump’s campaign quickly responded, according to the letter posted on Twitter. The Oct. 15, 2018, letter from Megan Newton, a Washington-based attorney who has represented a GOP super PAC in the past, pledges that the Trump campaign “will not use Prince’s music in connection with its activities going forward.”

Neither Newton nor the Trump campaign immediately responded to messages about why that promise was apparently broken Thursday.

Like all the musicians who have tried to disentangle their work from Trump’s political bombast, the legal recourse is murky for Prince’s estate.

As The Washington Post’s Travis M. Andrews reported in 2016, politicians generally have the right to use songs at rallies as long as they get a “blanket license” for the artist’s entire catalogue from ASCAP and BMI, which license songs. Artists can opt out of BMI licenses, though, and others have sued under the theory that it’s false advertising to implicitly suggest they support the candidate by using their music.

“Even if Donald Trump has the ASCAP right to use a Neil Young song, does Neil have the right to nevertheless go after him on right of publicity? I say he does,” Lawrence Iser, a copyright attorney, told Rolling Stone after Trump used Young’s “Rockin’ in the Free World” to help announce his candidacy in 2015.

During his career, Prince battled fiercely to maintain tight control over his music rights. And Prince’s personal politics would seem an unlikely match for the GOP president, who used his Thursday night rally to once again slam refugees and former vice president Joe Biden. As the Hill reported, Prince supported the Black Lives Matter movement before his death and was close with President Barack Obama, even playing a secret show at the White House in 2015.