Judges in Atlanta illegally impose “pay-or-jail” sentences on poor people, requiring them to pay a set fine or, if they’re unable to pay, to spend time in jail, according to a legal advocacy group.

The Southern Center for Human Rights on Monday sent a letter to Atlanta Mayor Keisha Lance Bottoms and Atlanta Municipal Court Chief Judge Christopher Portis saying that imposing such sentences on people who cannot afford to pay violates the U.S. Constitution. The letter threatens legal action if the practice isn’t ended.