Credit: Michael N. Todaro/FilmMagic

Bono fights fearlessly for humanitarian causes around the world, but there's one thing that really petrifies him: Alicia Keys!

At the premiere of the Showtime documentary Keep a Child Alive with Alicia Keys on Tuesday in New York, the Associated Press reports that the U2 frontman interrupted Keys' speech and said: "She's scary, isn't she? She's scary!" 

Bono, 51, added that the mom of 13-month-old Egypt and wife to Swizz Beatz has "lioness energy."

"I was terrified the moment I met her. I was shaking in my boots," the Irish musician and philanthropist said. "I was very moved by her singing of course, but what was interesting was the hard questions afterwards, and I think it's those hard questions that she asks that lead her." 

Keys, 30, started her charity, Keep a Child Alive, in 2003. It assists those affected by HIV/AIDS in Africa and India. She hopes her documentary reaches out to all those who want to help, including those unable to travel to Africa. 

Keep a Child Alive with Alicia Keys airs on the cable television Showtime channel on Dec. 1, which is World AIDS Day.