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 reports. 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 right here, arrives two days before U.S. District Judge George King was set to rule on the matter.

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