Karmin graced Lollapalooza with their quick rhymes and melodic singing over the weekend, but the duo were the recipients of a sweet surprise when Skylar Grey literally jumped on the mic behind them during the pair's rendition of 'I Need a Doctor.'

On Sunday (Aug. 7), Karmin was backstage at the Billboard Lollapalooza lounge performing an acoustic set, including the Dr. Dre and Eminem hit. During the second chorus, Grey seemed to leap out of nowhere and onto the red couch behind the singing duo to harmonize on the vocals. While Amy Heidemann is known for her wide-eyed expressions, fans could see the shock on her face as Nick Noonan maintained a cool demeanor.

Karmin are currently recording an album, but still posting covers on YouTube while in the studio. The duo exploded on the scene with their rendition of Chris Brown's ‘Look At Me Now,’ and wowed audiences with their version of Lil Wayne‘s ‘6 Foot 7 Foot.’ Most recently, the Massachusetts natives put their enthusiastic spin on a cover of LMFAO‘s chart-topping single, ‘Party Rock Anthem.’

Watch Karmin Perform 'I Need a Doctor' at Lollapalooza