WASHINGTON (AP) — The Supreme Court on Friday struck down part of a federal law intended keep people convicted of repeated violent crimes in prison longer.

The justices ruled that a catchall phrase in the Armed Career Criminal Act defining what crimes make a defendant eligible for a longer prison term is too vague.

The court sided with Samuel James Johnson, who pleaded guilty to federal weapons charges in 2012. Johnson was sentenced to 15 years in prison — five more than he otherwise would have gotten — because of his prior convictions.

That law lists burglary, arson, extortion and the use of explosive as specific categories of previous crimes that can lead to a longer sentence. But it also says a violent felony is a crime that “otherwise involves conduct that presents a serious potential risk of physical injury to another.”

The court initially agreed to hear Johnson’s case to decide whether his prior conviction for possessing a sawed-off shotgun qualified as a violent felony under the enhanced sentencing law.

But two months after hearing the case, the court ordered another round of arguments over whether the law’s catchall phrase was so vague as to be unconstitutional.

Six justices agreed that the phrase is unconstitutional. Writing for the court, Justice Antonin Scalia said using “so shapeless a provision to condemn someone to prison for 15 years to life” violates the Constitution’s guarantee of due process.

Justices Anthony Kennedy and Clarence Thomas agreed only on the outcome, saying they would find that Johnson’s conviction for possession of a sawed-off shotgun does not qualify as a felony under the law.

Justice Samuel Alito was the lone dissenter. He said the language of the law was clear enough and noted that it’s similar to dozens of other state and federal laws.

The justices have struggled with this phrase for years. Scalia had called on his colleagues to rule it unconstitutional since 2011.

The Armed Career Criminal Act makes defendants eligible for longer prison terms if they have three prior convictions for crimes that are either violent felonies or serious drug offenses.

According to federal authorities, Johnson is a white supremacist who formed the Aryan Liberation Movement. He was arrested in 2012 for taking part in a plan to attack the government, minorities and others, the government said.