Gambling regulators in Washington state have ordered eSports game developer Valve Corporation to clamp down on ‘skin’ betting or face the consequences.

On Wednesday, the Washington State Gambling Commission issued a statement saying it had warned Valve to “immediately stop allowing the transfer of virtual weapons known as ‘skins’ for gambling activities through the company’s Steam Platform.”

Skins are purely cosmetic add-ons to eSports weapons that can be purchased with virtual currency or won via achieving certain game benchmarks. While they in no way affect game play, certain skins can be highly prized among game fans, and a lucrative side business of reselling skins via the Steam marketplace has developed.

Some third-party sites allow players to wager skins on the outcome of eSports matches, while other sites offer the ability to wager skins on purely random games of chance.