Due to a medical emergency, R&B crooner D'Angelo has been forced to cancel several concert dates including an appearance at Outside Lands on Friday, officials have confirmed.

D'Angelo was due to perform at the House of Blues tonight (Aug. 7), but an executive at the popular club relayed a message from the singer's agent that indicated he was too sick to perform. "It's serious, but he should be OK," they said, according to the Chicago Tribune. He also canceled a Tuesday show in Minneapolis and a stop in San Diego on Saturday.

That left a scheduled Friday appearance at Outside Lands in San Francisco unlikely, and a representative from the festival has now confirmed the cancellation.

"He expresses his sincere regret due to this unfortunate circumstance," the statement reads. The nature of the singer's medical emergency is unclear.

Taking his place in San Francisco will be legendary party band Chic, featuring Nile Rodgers.

"We hope that D'Angelo has a speedy recovery, and we are grateful to Nile Rodgers and Chic for stepping in at this late hour," says Allen Scott of Another Planet Entertainment, which is producing the festival. "Chic is still one of the greatest live bands around and I can guarantee that they are going to bring the party to Outside Lands on Friday."