Nearly 4,000 people have signed an open letter calling on Christie’s New York to cancel an upcoming sale dedicated solely to art created with artificial intelligence (AI)—the first of its kind for a major auction house—over concerns that the programs used to create some generative digital pieces are trained on copyrighted work and exploit human artists, CNN reports.

The online letter began circulating Saturday, the day after the Christie’s Augmented Intelligence sale was announced.

The sale, which the auction house expects will bring in more than $600,000, includes work by artists Refik Anadol, Harold Cohen, Holly Herndon and Mat Dryhurst, Alexander Reben, and Claire Silver.

The more than 20 lots in the sale span five decades, and roughly one quarter are digitally native works like non-fungible tokens (NFTs), according to Christie’s.

Examples of other works in the auction include light boxes and screens, as well as sculptures, paintings,, and prints.

Bidding is scheduled to begin February 20 and run until March 5.

At publishing, the letter calling on Christie’s to halt the auction had 3,936 signatures attached. The authors largely take issue with the AI models used to create some of the works in the sale, which they say were trained on copyrighted works without their creators’ authorization.