Blake Shelton surprised unsuspecting country music lovers at Country Jam 2016 when he invited girlfriend Gwen Stefani to crash his headlining set on June 18. During his performance — which was also on his 40th birthday — Stefani and Shelton performed their duet “Go Ahead and Break My Heart” together before Stefani took the stage alone for No Doubt’s “Hella Good.” That’s when the country star knew he’d made a mistake.

In an interview on Chelsea Handler’s Netflix series Chelsea, Shelton now admits that inviting Stefani to the stage during his set was the worst thing he could have possibly done.

“I’m doing my normal, you know, what I think is rockin’. Standing up there with my acoustic guitar in front of my stand and singing my songs,” Shelton tells Handler. “I had my band work up one of her songs, ‘Hella Good.’ And I thought ‘Man, that’d be so cool to have her come out and surprise.’ It’s the worst thing I could have done because all it did was make me look terrible.”

“Literally 40,000 were jumping up and down,” he continues. “I mean, that’s probably what it’s like at real concerts, but I was like ‘Whoa. What’s going on?’ People were so excited to see her.”

While Shelton and Stefani’s duet stole the festival, it was his girlfriend’s pop-rock hit that had the crowd energized.

“This is fun!” Shelton said midway through his Country Jam set, before Stefani stormed the stage. “Y’all have been so freakin’ nice to me on my 40th birthday and I guess if I had one birthday wish for me personally … I wish I could rock. I wish I was one of those type of artists that could rock but I can’t.”

“For my birthday I wish I could hear an artist who could rock, that’s my birthday wish,” he teased.

Stefani was happy to grant Shelton’s birthday wish as she took to the stage, shouting, “You want some of this?” The No Doubt singer then played “Hella Good,” urging fans to jump up and down for her man’s birthday, and they obliged.