“Soft Kitty” has been a longtime source comfort for Sheldon Cooper on “The Big Bang Theory,” but now the song is at the center of a legal headache for CBS and “Big Bang” producer Warner Bros.

On Monday, a lawsuit was filed by the daughters of Edith Newlin, a New Hampshire teacher who originally wrote a poem about a “soft kitty” in the 1930s. The heirs of Newlin, who died in 2004, claim in the copyright infringement suit that “Big Bang” has used the lyrics to the song without permission, according to the Associated Press.

According to the complaint, the hit comedy has used Newlin’s lyrics — which begin “soft kitty, warm kitty” — on at least eight episodes of the series without permission. The lawsuit, seeking unspecified damages from the sitcom’s producers and distributors, also says the lyrics have been used in the series’ merchandising with shirts, sweatshirts, pajamas and more products featuring the lyrics.

According to the AP, the lawsuit states: “The Soft Kitty Lyrics are among the best-known and most popular aspects of ‘The Big Bang Theory.’ They have become a signature and emblematic feature of the show and a central part of the show’s promotion.”

Additionally, the suit states that besides failing to credit Newlin, the defendants have sometimes made it appear as if the lyrics were written by the show’s producers. The suit maintains that Warner Bros. sought permission in 2007 from Willis Music Co., a company that previously published the lyrics in a book titled “Songs for the Nursery School.” According to Newlin’s daughters, Willis Music Co. authorized the use of lyrics without consultation or permission.

The lawsuit goes into great detail on the show’s use of the tune, even mentioning that the “Soft Kitty” lyrics have been sung by large audiences, led by “Big Bang” producers and actors, at at least three Comic-Con conventions.

One of Newlin’s daughters, Ellen Newlin Chase, first learned of “Big Bang’s” use of “Soft Kitty” in August 2014 when she was researching her mother’s history and came across a blog post regarding the show.

CBS did not immediately reply to Variety‘s request for comment. Warner Bros. declined to comment.