Music fans who skipped out on Ray Benson's birthday party on Tuesday (March 14) during SXSW are now kicking themselves for missing a surprise appearance from Willie Nelson.

Nelson took the stage unannounced around 9:30PM at the showcase in Austin, Texas, after an already impressive set by both the Avett Brothers and Asleep at the Wheel. According to Austin360, Nelson played close to a dozen songs including "Whiskey River" and "On the Road Again" as well as "Still Not Dead" from his upcoming album God’s Problem Child, due out this spring. Highlights included a cover of Townes Van Zandt’s "Pancho & Lefty" with Benson — which you can watch in the video above — and "Roll Me Up and Smoke Me When I Die."

Benson’s 66th birthday party featured performances from the Avett Brothers, Randy Rogers, Sunny Sweeney, Wade Bowen and more. Last year, George Strait made a surprise appearance during the event while in 2015 Gary Clark Jr. attended.

Benson's birthday concert is an annual SXSW staple, and the show raises money for the Health Alliance for Austin Musicians (HAAM). According to Billboard, since 2005 the event has raised $4.9 million to help provide health care for more than 4,400 musicians.

Nelson recently canceled several concerts due to illness, and his appearance on Tuesday showed the country legend in good spirits while holding his own on stage. The singer is set to release his forthcoming album on April 28. The album includes 13 new songs, and according to a press release, God’s Problem Child “reflects on mortality while marveling at the beauty and absurdity of it all."

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