The hit CBS sitcom "The Big Bang Theory" has reaped in success season after season; but now, it looks like the show has encountered a major bump in the road since it is being sued over its "Soft Kitty" song.

"Soft Kitty" is the favorite lullaby of lead star Sheldon Cooper (played by Jim Parsons). His friend Penny (played by Kaley Cuoco) would sing it to him whenever he needs comforting. The song goes: "Soft kitty, warm kitty, little ball of fur. Happy kitty, sleepy kitty, purr purr purr."

It was first sung on the episode "The Pancake Batter Anomaly" in 2008, then repeated on "The Vegan Renormalization," "The Electric Can Opener Fluctuation," "The Adhesive Duck Deficiency," "The Cruciferous Vegetable Amplification," "The Rhinitis Revelation," "The Proton Resurgence" and "The Anxiety Optimization" episodes.

According to Deadline, the lyrics for the song "Warm Kitty" were actually written back in 1937 by the late poet Edith Newlin. Her daughters Margaret Perry and Ellen Chase are now demanding that CBS pay up.

They said in the suit that they filed in federal court in Manhattan that, "The Soft Kitty lyrics have played a prominent role in the development and portrayal of one of the central characters in the program."

When asked for comment, Warner Bros., which produces the show, declined to give any. Meanwhile, Willis Music, the music publisher that bought the book nearly 80 years ago said that the song is properly and legally licenses to Warner Bros.

"In 1937, we published a book called 'Songs for the Nursery School,' and we sold tens of thousands of copies," wrote Willis Music owner Kevin Cranley on the company's website. "It is a hardbound book of over 150 songs for children. The book was written by Laura Pendleton MacCarteney. In that book on page 27 is 'Warm Kitty.' Warner Brothers and I worked together to secure the rights for the show 'The Big Bang Theory' and they have been using the song ever since. The writers wanted the song because one of them remembered it as a child. They also wanted to slightly change the words and I'm really not sure of the reason for that change."

The original lyrics in Newlin's version go: "Warm kitty, soft kitty, little ball of fur; Happy kitty, sleepy kitty, purr! purr! purr!" The music used for the improvised lyrics in "The Big Bang Theory" was adapted from an English folk tune.