Kendrick Lamar had the honor of being the only rap guest appearance on Eminem’s 2013 album The Marshall Mathers LP 2, but he had to earn it by passing a test. A new interview with Ed Sheeran finds the UK singer recalling a story about Eminem’s practices that he heard from Rick Rubin, who worked extensively on MMLPII. According to Sheeran, Em made sure that when Kendrick came to write his verse, he didn’t have any help.

“Eminem heard that Kendrick Lamar was the best rapper and he invited him to the studio to get him on a song,” Sheeran revealed. “He arrived and Kendrick came with all his mates and Eminem said, ‘I just want you in the studio, just you on your own and then my engineer is gonna come in and then record you doing it, but your mates aren’t allowed in’.”

It wasn’t until later that Eminem revealed why he had this policy in place. “Everyone came in to listen to it and Eminem said that he did it to test Kendrick because he thought he had a ghostwriter.” Apparently, after he saw Kendrick write his “Love Game” verse with his own eyes, he was willing to call Kendrick “the best.”

Listen to Sheeran tell the story below. It starts at the 8-minute mark.