The Big Bang Theory’s popular “Soft Kitty” song was hit with a copyright lawsuit since the series never obtained permission to use it. Introduced by Sheldon (Jim Parsons) in the sitcom, he claimed that it’s a children’s song that his mother usually sang to him when he was ill. Throughout The Big Bang Theory’s 12-season run, the genius but socially-inept character asked some of his friends to perform the tune for him.

In The Big Bang Theory season 1, episode 11, “The Pancake Batter Anomaly,” “Soft Kitty” debuted when Sheldon forced Penny (Kaley Cuoco) to sing the song when he caught the cold. Knowing that it comforted her friend, Penny took it upon herself to sing it a few more times later, whenever Sheldon was feeling down in an attempt to cheer him up. By season 8, Leonard (Johnny Galecki) and Penny used it as a lullaby in the hopes of putting their friend to sleep after refusing to go to bed. The last time “Soft Kitty” was performed in the show was in season 10 when Amy (Mayim Bialik) sang the song to her stressed boyfriend in three different languages - English, German, Mandarin.

Young Sheldon - The Big Bang Theory’s prequel spin-off - has actually shown Mary (Zoe Perry) singing this to the boy genius, backing up the character’s claim in the original series. While neither show has confirmed where it originated, the main idea was that it was an original creation for The Big Bang Theory. However, the song’s tune came from a 19th century Polish lullaby called “Wlazł kotek na płotek” about a kitten on a fence, written by Oskar Kolberg and composed by Stanisław Moniuszko. The contemporary version of the song, on the other hand, is known as “Warm Kitty”, written by Edith Newlin. It appeared in a book titled Songs for the Nursery School first published in 1937 by the Kentucky-based Willis Music. This was where the controversy surrounding The Big Bang Theory’s “Soft Kitty” began.

In December 2015, Newlin’s estate headed by her daughters Margaret Perry and Ellen Chase filed a copyright lawsuit against Warner Bros., Chuck Lorre Productions, CBS, Turner Broadcasting, and Fox for the supposedly illegal use of their mother’s work. Aside from using the song on the show, CBS also had a string of tie-in merchandise including Soft Kitty T-shirts, plush toys, key chains, coffee mugs, and hoodies - taking advantage of its popularity.

According to Perry and Chase, they believed they were owed compensation for The Big Bang Theory’s use of the “Warm Kitty” lyrics, emphasizing that they held its copyright and did not authorize anyone to use said lyrics. When asked why it took long for them to raise the issue given that “Soft Kitty” had been used in The Big Bang Theory since season 1 (with the song first being used in the show in 2008), they claimed that they didn’t watch the sitcom, so they never knew that it was using their mother’s work until they came across a blog post about it. Meanwhile, Willis Music countered by insisting that the song was properly and legal licensed to Warner Bros. for The Big Bang Theory.

More: Why The Big Bang Theory’s Sheldon’s Original Name Was Changed