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A documentarian making a film about the song “Happy Birthday’ may have just crashed the party of Warner/Chappell Music, which has lucratively held the tune’s copyright since 1935.

After filing a lawsuit two years ago claiming that the world’s most popular song shouldn’t be under copyright, filmmaker Jennifer Nelson submitted new evidence yesterday that could ultimately be irrefutable, Ars Technica . The so-called “smoking gun” is a 1927 songbook containing the song’s lyrics that precedes Warner/Chappell’s copyrighted version by eight years.

Warner/Chappell, which is one of the world’s largest music publishers and a division of the Warner Music Group, currently licenses the tune out for more than $2 million per year, according to Ars Technica.

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Nelson, who had to pay $1,500 in order to use the song for her film entitled “Happy Birthday,’ is seeking a refund, and is also representing a class of plaintiffs with similar grievances.

The 1927 Everyday Song Book, recently unearthed by prosecutors, which Warner/Chappell “mistakenly” failed to produce during the discovery phase ending last July, features the “Birthday Song’ without a copyright notice — meaning that, according to the 1909 Copyright Act, it had therefore been “interjected irrevocably into the public domain.”

“Happy Birthday’ was originally co-written by Kentucky sisters (and schoolteachers) Patty and Mildred Hill, and first published in 1893 by Clayton Summy, a company later purchased by Warner/Chappell. The prosecution’s filing, which can be viewed in full , arrives two days before U.S. District Judge George King was set to rule on the matter.

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