A Thai military court on Friday sentenced a 67-year-old man to 18 months in prison for writing insults against the country’s monarch on the wall of a shopping mall restroom, a lawyer said.

The military court in Bangkok found Opas Charnsooksai guilty of lese majeste the crime of defaming, insulting or threatening the monarchy, for writing graffiti on the restroom wall in a Bangkok mall last October.

Thailand’s lese majeste law is the harshest in the world, providing for jail terms of three to 15 years. After invoking martial law and staging a May 22 coup, the military has intensified a crackdown on criticism of the monarchy.