The world’s most popular song might enter the public domain.

Filmmakers working on a documentary about ‘Happy Birthday to You’, who are in the midst of suing Warner/Chappell for the rights to use the song in their film, have found new evidence that could invalidate the 80-year-old copyright.

According to their attorneys, the plaintiffs have found the “proverbial smoking gun”: a book of children’s songs that contains the lyrics to ‘Happy Birthday’, predating Warner’s copyright claim. The researchers originally found a blurry copy of the book from 1927 in Warner/Chappell’s digital library, and later found a copy that was published in 1922. The copyright registration that Warner/Chappell relies on was made in 1935.

If Warner/Chappell loses the case, it would reportedly lose $2 million a year in revenue if it could no longer license the song for use in film and TV. The initial class action lawsuit against the filmmakers was filed by Warner/Chappell in 2013 after director Jennifer Nelson was told she’d have to pay $1,500 to use the song in her documentary.

One of the attorneys for the plaintiffs, Randall Newman says Warner/Chappell “should admit defeat but they won’t because too much money is at stake.” Warner/Chappell’s attorneys were unavailable to respond. [via The Hollywood Reporter]