President Donald Trump’s recent appearance at the U.S. Open sparked multiple loud boos, jeers and occasional cheers from the crowd. And one American studies professor says there’s a lot to take away from the icy reception.

Trump made his first appearance at the U.S. Open as a sitting president on Sunday afternoon to watch the match between Jannik Sinner and Carlos Alcaraz. The last time Trump attended the tennis tournament was in 2015, when he was a candidate for president.

There were several times on Sunday when Trump didn’t exactly receive a resoundingly warm welcome at Arthur Ashe Stadium in his native New York City borough of Queens.

The president faced a mix of boos and cheers when he briefly appeared at the stadium about 45 minutes before the match began — and when the stadium was still mostly empty, the Associated Press reported.

Trump was later met with loud boos and some cheers when he was shown on the jumbotron during the national anthem. And he faced more boos for a longer period of time when he was featured again on the big screen at the end of the first set, the AP reported.