Fans of CBS comedy "The Big Bang Theory" are familiar with the song, "Soft Kitty," which, as main character Sheldon Cooper said, is a lullaby for when one is sick.

The song was so popular that Time mentioned it's even been part of the TBBT merchandise with shirts and fan contests.

Nine seasons later, a pair of sisters by the name Ellen Newlin Chase and Margaret Chase Perry have filed a lawsuit against CBS and Warner Bros for the use of the lyrics, who they said was written by their mother, Edith Newlin. In 1937, Newlin published a book called "Songs for the Nursery School," which included the words of Sheldon's lullaby, but with "soft/warm" and "happy/sleepy" interchanged.

The lawsuit said that the original read, "Warm kitty, soft kitty, / Little ball of fur, / Sleepy kitty, happy kitty, / Purr! Purr! Purr!"

The sisters claimed that the book publisher, Willis Music (also a defendant in the case) registered the collection with the Copyright Office in 1937 and had it renewed in 1964. According to copyright laws, this also serves to register and renew Edith Newlin's copyright of the lyrics.

The author died in 2004, and her daughters claimed that after her death, they have become the sole owners of the copyright, and when the producers of the sitcom sought to use the song, they only negotiated with Willis Music, and not with them. Therefore, the sisters are seeking damages from the sitcom, arguing that they deserve compensation for the lyrics.

The suit also stated that failing to credit their mother aside, the defendants have made it seem like the lyrics were written by the show's producers, adding that the lyrics have been sung by large audiences, led by the show's producers and actors, in at least three Comic Cons.

The lawsuit went on to say, "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."

But why did it take nine seasons for Newlin's daughters to hit the show with a lawsuit? According to Variety, Ellen Chase first learned of the use of the song only in August 2014, when she was researching her mother's history. It was during the said research, where she came across a blog post regarding the show.

CBS and Warner Bros have not commented on the issue.