A federal judge’s ruling this week that the entry fees paid to play on the fantasy horse racing website Derby Wars are wagers—the first time a U.S. court has taken that position—could be a warning signal to the booming fantasy sports industry, lawyers say.

The decision was issued by Judge James Otero of the U.S. District Court for the Central District of California in a case pitting Louisville, Kentucky-based Horse Racing Labs, the parent company of Derby Wars, against The Stronach Group, a collection of horseracing associations operating at tracks including Santa Anita Park in Southern California, Golden Gate Fields in San Francisco and Pimlico in Baltimore, Maryland.