Demi Lovato almost gave her number one fan a heart attack. 

Nathan, who calls himself Lovato's biggest fan, thought he was recording a fan video to be played for her during an interview at a local station when he got the surprise of a lifetime. 

As he was talking about how the singer changed his life and made him a better person, Lovato casually walked into the studio. 

Watch his reaction to seeing his idol here: