A South Korean law firm representing three members of K-pop boy band EXO says the singers are pursuing legal action against their longtime label and management agency SM Entertainment over contractual issues related to “slave contracts.”

In a press release, Lin Law Firm claims it has represented K-pop stars Baekhyun, Chen and Xiumin — who are members of EXO and also perform together in a splinter trio unit named EXO-CBX — since March over pay and contract disputes with SM Entertainment, who debuted EXO in 2012 and are home to acts like TVXQ!, Girls’ Generation, SHINee, NCT and aespa.

Baekhyun, Chen and Xiumin (whose full names are Byun Baekhyun, Kim Jongdae and Kim Minseok, respectively) claim SM has shown a lack of payment transparency and required unreasonably long contracts extending beyond 12 years, according to a five-page document reviewed by Billboard that was sent by Lin Law Firm attorney Lee Jaehak.

The firm alleges that despite the trio signing exclusive, long-term contracts with SM, the K-pop company has not provided full data about the artists’ payments as they recently requested. Lin Law adds that the artists have always trusted SM’s payments despite Korean law requiring entertainment companies to provide updates on payment settlements twice a year.

Lin Law Firm also claims that SM has used its position in the K-pop market to force artists to sign with the company for longer than the industry standard seven years — deals it calls “slave contracts.” The firm says that SM automatically extends artist contracts by three years if the artist works overseas, which applies to Chen and Xiumin, as the two were originally part of the China-focused group EXO-M. Meanwhile, Baekhyun has released solo music in Japan and was a part of the U.S.-focused, Billboard 200-topping “supergroup” SuperM.

The final point in Lin Law’s document includes an apologetic message to fans and a pledge to resolve the dispute.

SM Entertainment has not responded to Billboard‘s request for comment. Baekhyun, Chen and Xiumin have not publicly commented on the matter, either.

Alleged “slave contracts” are a historically sensitive spot for SM Entertainment. In 2009, three of the original five members of boy band TVXQ! asked Korean courts to examine their 13-year contracts, citing extreme length and worries about payment distribution. Over the course of three-plus years of legal battles, singers Kim Jaejoong, Park Yoochun and Kim Junsu won the right to work independent of their SM deals and formed a new boy band named JYJ. By November 2012, the two parties mutually agreed to terminate the SM contracts, which would have expired in 2016 at the earliest.

As a result, South Korea’s Fair Trade Commission created a rule in 2010 that did away with so-called “slave contracts,” requiring entertainment companies to sign individuals for a maximum of seven years initially. Lin Law does not explain how SM could legally surpass the rule.

As members of EXO, Baekhyun, Chen and Xiumin helped lead K-pop’s international expansion with the group’s dual focus on releasing music in both Korean and Chinese. In April 2015, EXO set a new record for the largest sales week for K-pop artists in America at the time when its Exodus album sold 6,000 copies, according to Luminate. It held that record until late 2016.

EXO has scored five No. 1s on Billboard‘s World Albums chart, while EXO-CBX also topped the chart with its debut record Hey Mama! from 2016. All three members have also released solo albums, with Chen releasing a new song, “Bloom,” on May 30 through SM Entertainment. After several EXO members fulfilled their South Korean military duties, the group reunited for the first time in years. SM has confirmed the group would release a new studio album together this year.