Civil rights advocates denounced New Jersey’s prison system on Monday after learning that the book The New Jim Crow, about race and mass incarceration in the U.S., has been banned from some of the state’s prisons.

“The DOC—and every player in the criminal justice system, from police officers and prosecutors to judges and legislators—must take affirmative steps to reduce our state’s shameful racial disparities,” wrote Tess Borden and Alexander Shalom, attorneys with the state’s American Civil Liberties Union (ACLU) chapter, in a letter to the New Jersey Department of Corrections. “The ban on The New Jim Crow does precisely the opposite and is a step backwards instead.”

“In its worst light, it looks like an attempt to keep impacted people uninformed about the history of the very injustice that defines their daily lives.”—ACLU-New Jersey

The ACLU accused the prison system of violating the First Amendment by withholding the book, written by Michelle Alexander and subtitled “Mass Incarceration in the Age of Colorblindness,” and argued that there is no legitimate reason to ban the award-winning work.

Under New Jersey’s own regulations, prisons are allowed to ban material that has been shown to hinder the security of facilities—including writings on lock-picking, weapon-making, and materials that could “incite violence”—but The New Jim Crow is far from fitting this description, said the attorneys.