U.N. rights experts asked Nigeria on Monday to release a 22-year-old singer who was condemned to death over an allegedly blasphemous song, and said the sentence broke international law.

Yahaya Aminu Sharif was sentenced last month by a sharia court in Kano, the commercial hub of Nigeria’s mostly Muslim north, after he performed the song and shared it on WhatsApp.

“Music is not a crime,” read a joint statement from the group of U.N. rapporteurs.

“Application of the death penalty for artistic expression or for sharing a song on the internet is a flagrant violation of international human rights law, as well as of Nigeria’s constitution,” said Karima Bennoune, special rapporteur on cultural rights.

The rights experts said Nigeria should overturn the death sentence and guarantee the singer’s safety while he launched an appeal. Protesters enraged by the song burned down Sharif’s family home on March 4.

There was no immediate reaction from the judicial authorities in Kano, which runs sharia alongside civil courts.

The state’s justice system has been in the spotlight since a sharia court also sentenced a 13-year-old boy to 10 years in prison last month after he was accused of making blasphemous statements during an argument.

The head of Poland’s Auschwitz Memorial has written to Nigeria’s president asking him to pardon the boy, Omar Farouq, and offering to serve part of the jail term himself.

Kano’s sharia courts are active, but death sentences for blasphemy are unusual and the most recent, handed down in 2015 to nine followers of the Tijani Muslim sect, have yet to be carried out.