BERLIN (AP) — A Bavarian court found a folk singer guilty Friday of violating Germany’s strict child labor laws for having his 4-year-old son perform on stage with him during an evening concert.

Angelo Kelly, 39, was fined 3000 euros ($3,600) by the Hassfurt administrative court for the 2019 appearance at an open air summer concert with his son William, who sang “What a Wonderful World.” The boy is the youngest of Kelly’s five children.

“During this performance, the child William stood for at least 30 minutes on the stage where he played along, sang along and interpreted his own song,” the court said, according to the dpa news agency. “This is considered work by the Youth Labor Protection Act.”

Kelly, who is popular in Germany, said on his Facebook page that he would appeal the verdict.

“As a father, the well-being of my children is the most important thing for me,” he wrote. “William was and is never obliged to appear with us at our shows. If he did it, it was because he wanted to.”

Children ages 3 to 6 can take part in musical performances under German law for up to two hours a day, but only with official approval of an exception and only between the hours of 8 a.m. and 5 p.m., dpa reported. William, who is now 5, appeared on stage until 8:20 p.m., according to authorities.

Kelly’s attorney, Julian Ackermann, blasted the ruling, saying that interpreting a “brief appearance on the stage in the presence of one’s mother and siblings at a concert given by one’s own father” as child labor was legally wrong and “far removed from the facts.”