Country legend Willie Nelson is too ill to perform at tonight’s Outlaw Music Festival in Alpharetta, according to Ameris Bank Amphitheatre.

Nelson, 91, is the headliner for the festival, which kicks off tonight and begins at 5:30 p.m.

“We regret to inform you that Willie Nelson is not feeling well and, per doctor’s orders, has been advised to rest for the next four days,” the amphitheater said in an email to ticket holders. “He is expected to make a quick recovery and join the Outlaw Music Festival tour next week. In the meantime, Lukas Nelson and the Family Band, along with a few guests, will perform a special set include Willie’s classics and other songs.”

Bob Dylan, Robert Plant, Alison Krauss and Celisse will perform as scheduled, the venue said.

Nelson is also expected to be absent when the tour resumes Saturday in Charlotte, North Carolina, and Sunday in Raleigh. He could return Wednesday, when the tour hits Virginia Beach, Virginia.

Additional dates for the Outlaw Music Festival are planned into September. John Mellencamp will take part in later dates.

“This year’s Outlaw Music Festival Tour promises to be the biggest and best yet with this lineup of legendary artists,” Nelson said in a statement in February. “I am thrilled to get back on the road again with my family and friends playing the music we love for the fans we love.”