The Federal Trade Commission has sent a warning to Instagram “influencers”: Be more transparent when plugging a product.

The agency sent out 90 letters to celebrities and brands last month to remind them any endorsement on social media must be disclosed in compliance with the FTC's rules. Celebrities like Kourtney Kardashian, Scott Disick, Sean “Diddy” Combs, Allen Iverson, Naomi Campbell, and Jennifer Lopez were some of the stars who received the warning, along with brands such as Adidas, Chanel, Puma, Saint Laurent, and Popeyes Louisiana Kitchen.

WWD obtained the letters, which were reportedly intended to “educate” celebrities on how to post about a product without committing any violations.

The documents read:

The FTC’s Endorsement Guides state that if there is a ‘material connection’ between the endorser and the marketer of a product—in other words, a connection that might affect the weight or credibility that consumers give the endorsement—that connection should be clearly and conspicuously disclosed, unless the connection is already clear from the context of the communication containing the endorsement. Material connections could consist of a business or family relationship, monetary payment, or the provision of free products to the endorser.

According to WWD, the warnings also stated a paid ad must clearly expressed, meaning the disclosure should use “unambiguous language” that “stands out.” No-nos included posting disclosures at the bottom of a caption that could only be read after clicking the “more” link, and burying the disclosure in multiple hashtags.

Of course, if the celebrity is simply praising a product he/she was not paid to mention, they don’t have to worry.

You can check out the full list of celebrities and brands that received the warnings at WWD.