In a March 20 letter, the Federal Trade Commission put on notice the manufacturer of the popular Muscle Milk line of protein shakes: If a celebrity gets paid for a promotional post on Instagram, that business relationship would need to be disclosed.

The FTC highlighted an Instagram post by Nina Agdal, in which the model poses in the corner of a boxing ring with boxing gloves and two cartons of Muscle Milk at her feet. The caption of the Instagram post: “Spent my morning in the ring with @MuscleMilk @BoxingNapoleon,” followed by emoticons of a trophy and a flexed bicep.