On The Big Bang Theory, the famed “Soft Kitty” lullaby may have soothed Sheldon into a peaceful slumber, but right now, it’s giving show producers a legal headache.

Warner Brothers, CBS, and others involved with the hit sitcom (which saw whopping ratings during its recent “coitus” episode) are being slapped with a lawsuit over the repeated use of the song, which has been featured in show merchandising and even a fan singing contest.

According to the claims, the song was penned as the poem “Warm Kitty” by a late schoolteacher named Edith Newlin around 1933, and then published in 1937 in a book called “Songs for the Nursery School.” Edith Newlin’s daughters, Ellen Newlin Chase and Margaret Chase Perry, are suing the companies for copyright infringement, saying that their mother was never credited and permission to use the song was never sought.

Here’s the original song. The Big Bang Theory flipped the words “warm kitty” and “soft kitty” in its rendition.

The plaintiffs claim that in 2007, Warner Bros. Entertainment “worked out a deal” with the songbook publisher, Willis Music, to use “Warm Kitty” in the show, but Willis Music didn’t have the right to grant that permission. Instead, the publisher was granted a one-time use of the poem, which it paired with public domain music. Edith Newlin passed away in 2004.

Here’s a 1993 recording of “Warm Kitty” made for Play School, an Australian children’s television program.

And here’s Penny (Kaley Cuoco) singing the song to Sheldon (Jim Parsons) in The Big Bang Theory.

The lawsuit has ignited a discussion in Reddit’s Television community about the public domain—a realm of works whose copyright has expired and are therefore free for anyone to use, remix or reimagine.

In a way, Warner Bros. can thank Disney for its legal problems.

This three-minute segment from Adam Ruins Everything sums up the issue rather beautifully.

If “Warm Kitty” was published a decade earlier, Sheldon and the show producers could rest easy knowing that the song had entered public domain. But over the years, Disney has changed the course of the public domain system. 1928’s “Steamboat Willie,” the very first Mickey Mouse cartoon ever produced, was slated to become public domain in 1984.

Disney couldn’t let that happen so it lobbied Congress to extend copyright law to a term of 75 years. And the company lobbied for another extension again in 1998. And it’s would seem likely that it would do so again—and soon—as the works of 1923 are now set to enter public domain in 2018.

Many believe The Mouse’s squashing of the public domain is a hypocritical move because Disney has made billions in revenue from its movies based on public domain stories—Adventures of Huck Finn was based on Mark Twain’s 1885 book, Alice in Wonderland comes from Lewis Carroll’s 1865 tale and Frozen is the reimagining of Hans Christian Anderson’s 1845 Ice Queen, just to name a few.

But others see the law as fair.

Of course, if Warner Bros. would have asked for permission from the right entity in the first place, this might not be an issue.