ALPHARETTA, Ga. — Country great Willie Nelson has canceled his headlining appearance at an Alpharetta music festival tonight due to illness.

The 91-year-old singer was set to headline the Outlaw Music festival at Ameris Bank Amphitheatre. The show kicks of 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 including Willie’s classics and other songs.”

Bob Dylan, Robert Plant, Alison Krauss and Celisse will still perform.

Officials did not give details on Nelson’s illness.

The tour is supposed to resume Saturday in North Carolina, but Nelson is not expected to perform.