The Supreme Court ruled Friday that thousands of low-level drug dealers are ineligible for shortened prison terms under a Trump-era bipartisan criminal justice overhaul.

The justices took the case of Mark Pulsifer, an Iowa man who was convicted of distributing at least 50 grams of methamphetamine, to settle a dispute among federal courts over the meaning of the word “and” in a muddy provision of the 2018 First Step Act.

The law’s so-called safety valve provision is meant to spare low-level, nonviolent drug dealers who agree to plead guilty and cooperate with prosecutors from having to face often longer mandatory sentences.

Some courts had concluded the use of the word indeed means “and,” but others decided that it means “or.” A defendant’s eligibility for a shorter sentence depended on the outcome.

“Today, we agree with the Government’s view of the criminal-history provision,” Justice Elena Kagan wrote for the majority in the 6-3 decision that did not split the justices along liberal-conservative lines.

In dissent, Justice Neil Gorsuch referred to the First Step Act as possibly “the most significant criminal-justice reform bill in a generation.” But under the court’s decision, “thousands more people in the federal criminal justice system will be denied a chance—just a chance at” a reduced sentence, Gorsuch wrote, joined by Justices Ketanji Brown Jackson and Sonia Sotomayor.

Nearly 6,000 people convicted of drug trafficking in the 2021 budget year alone are in the pool of those who might have been eligible for reduced sentences, according to data compiled by the U.S. Sentencing Commission.

The provision lists three criteria for allowing judges to forgo a mandatory minimum sentence that basically looks to the severity of prior crimes. Congress wrote the section in the negative so that a judge can exercise discretion in sentencing if a defendant “does not have” three sorts of criminal history.

Before reaching their decision, the justices puzzled over how to determine eligibility for the safety valve — whether any of the conditions is enough to disqualify someone or whether it takes all three to be ineligible.

Pulsifer’s lawyers argued that all three conditions must apply before the longer sentence can be imposed. The government said just one condition is enough to merit the mandatory minimum.