A Spanish counter-terrorism law is stifling online creativity and artistic expression, Amnesty International warned.

The human rights organization highlighted instances in which Spanish authorities have used a law criminalizing the “glorification of terrorism” or “humiliating victims of terrorism” to persecute social media users and artists in ways that amount to “a chilling attack on freedom of expression,” according to the group. In 2017, 39 people were charged under Article 578 of the Spanish Criminal Code, which Amnesty charges is vague and ill-defined. One of those charged was Cassandra Vera, a 22-year-old student who made a Twitter joke about the assassination Franco confidant Luis Carrero Blanco—even though the niece of Blanco opposed the student’s prosecution. While Vera was convicted and given a suspended sentence of one year in prison, courts in Spain have rolled back some prosecutions under the law, and Vera’s sentence was overturned. They also shelved an investigation into a puppet show that allegedly supported terrorism, but Amnesty International warns that the law has created a climate of self-censorship and a fear of engaging in political satire. The European Union is implementing a directive on terrorism in September that includes language similar to the Spanish code, specifically that “glorification” of terrorism may be criminalized.