After Alex Goldschmidt proposed to Ross Girard in Los Angeles this weekend, they gathered with their loved ones to celebrate.

But never in partygoers' wildest dreams could they expect what happened next: a private serenade by Taylor Swift.

"There is one secret that I didn't trust any of you with," Goldschmidt told their friends, according to a video posted on his Instagram.

"There was a person who's not here, who sort of played a part in all of this," he said. "So I would like to welcome, and if you would give a warm welcome to, my friend Taylor."

That's when Taylor Swift -- THE Taylor Swift -- walked out from some back room to cheers and applause. No one was ready for it when she broke out into an acoustic rendition of "King of My Heart."

"This is from Alex, but sung by me," Swift said.

Watch the moment below, via Goldschmidt's Instagram page.

Goldschmidt told CNN he had reached out to Swift and her publicist to say he was going to propose to Girard, and to thank her for writing the song he was listening to when he decided to pop the question.

"She reached out wanting to be a part of it," Goldschmidt said via email Sunday. "It was all her idea."

It was the perfect way for Goldschmidt to tell Girard, "You Belong With Me." Of course, their love story is just beginning.

Source: CNN