The band was due to perform on Conan but they axed the slot to allow the singer to get better ahead of her appearance at a concert in Los Angeles on Wednesday night (01May13).
In a series of posts on her Twitter.com page, Williams writes, "Unfortunately we have to cancel our performance on Conan OBrien tonight. I am really sick today and want to be 100% for our show tomorrow night at The Wiltern in La.
"I'm sorry to let anyone down. Mostly Conan OBrien cause he's my Tv crush. Love you guys."