The Big Bang Theory season 9
The Big Bang Theory producers are in trouble over copyright issues related to Soft Kitty lyrics CBS

The Big Bang Theory's lullaby song Soft Kitty has landed CBS producers and Warner Bros in trouble. The daughters of Edith Newlin, a New Hampshire teacher who originally wrote a poem about a soft kitty in the 1930s, have filed a lawsuit against the producers.

Associated Press reported that The Big Bang Theory used the lyrics, which begins Soft kitty, warm kitty, without buying the rights. According to the lawsuit, the hit comedy used Newlin's lyrics in at least eight episodes. Newlin created the lyrics: "Warm kitty, soft kitty, little ball of fur, Happy kitty, sleepy kitty, purr! purr! purr!" in or around 1933.

According to the report, 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 its promotion.

The lawsuit, seeking unspecified damages from the sitcom's producers and distributors, also claimed the lyrics have been used in merchandising, including shirts, sweatshirts and pyjamas.

The legal case states that in 2007, Warner Bros. Entertainment and the show's other producers sought permission from Willis Music Co., a Kentucky-based company that published the lyrics in a book called Songs for the Nursery School, to use the song.

The suit says: "Willis Music authorized use of the lyrics without consulting or getting permission from Newlin's heirs even though the book makes clear on its acknowledgement page and where the lyrics appear that Newlin was the author of and owned the copyright to the lyrics."

The lullaby was introduced early in the series and characters like Penny and Mary Cooper have often sung it to comfort Sheldon Cooper, played by Jim Parsons. It has often been repeated through the course of the series, which is currently in the ninth season.

Warner Bros and The Big Bang Theory are yet to comment on this topic.