A lullaby that is a part of the hit CBS sitcom “The Big Bang Theory” is at the center of a copyright lawsuit. The AP reports that the heirs of a New Hampshire teacher who wrote a poem about a “soft kitty” about 80 years ago are accusing the comedy’s producers of copyright violation.

The suit was filed by the daughters of Edith Newlin, who reportedly wrote the poem in the 1930s. The lullaby, which is sung at times on the show as a way to comfort the character Sheldon Cooper (Jim Parsons), includes the lyrics “Soft Kitty, warm kitty.”

“The daughters’ lawsuit, seeking unspecified damages from the show’s producers and distributors, says the ‘soft kitty’ lyrics have been used in their entirety on at least eight episodes of the show since March 2008,” the AP reports. “It says the lyrics also have been used in merchandising, including being displayed in their entirety on T-shirts, sweatshirts, hoodies, pajamas, mouse pads, mobile phone covers, wallets, air fresheners, refrigerator magnets, singing plush toys and other products.”

A spokeswoman for CBS reportedly declined to comment.