A conservative nonprofit organization must hand over the names of its top contributors to the California attorney general, a federal appeals court ruled Tuesday in a legal blow for the group’s founders, brothers Charles and David Koch.

A three-judge panel for the U.S. Court of Appeals for the Ninth Circuit unanimously sided against the Americans for Prosperity Foundation in its challenge to the California attorney general’s disclosure requirement, collecting the names and addresses of the largest donors to the group. The nonprofit—which was founded in 2004 and has tea party ties—along with a number of other groups, had claimed the requirement burdened their First Amendment right to free association by deterring possible contributors.