The Obama administration took a small step on Thursday to prevent coal companies from fleecing taxpayers.

Until now, the biggest coal conglomerates were getting away with scamming the government by selling coal mined on federal land to their own subsidiaries for a discounted, below-market price. Since the government’s royalties from that coal are a percentage of the sale price, that meant the companies were paying lower royalties than they should have been. Forty-two percent of the coal produced in Wyoming’s Powder River Basin — the biggest coal-producing area in the U.S. right now — was being sold through these “captive transactions,” according to Public Citizen, a good government advocacy group.

On Thursday, the Interior Department issued a new rule that puts a stop to that practice.