Keith Richards was left bewildered when Christina Aguilera joined The Rolling Stones to perform a song during their new Martin Scorsese-directed concert film - because he had no idea who she was.

The guitarist looks a little puzzled on film as Aguilera takes the stage for a rendition of "Live With Me" in new rockumentary Shine A Light, and he freely admits he thought she was a fan invading the stage.

Richards says, "This girl came out and I was like, 'Who is that?' I had no idea. I'm still not sure who that is. She can sing, though. And she looked good."

-Christina Aguilera Pictures

(This news article provided by World Entertainment News Network)