The company that claimed a copyright on the song "Happy Birthday" has agreed to return $14 million collected in fees after a judge last year ruled its copyright invalid.

Warner/Chappell Music's copyright of the song "Happy Birthday" was declared invalid by a California judge in September of 2015, who wrote that the song, referred to in a suit as "the world's most popular song," should be in the public domain.

In papers filed in federal court, Warner/Chappell Music says it will pay up to $14 million to settle any licensing fees paid to use the song in the past.

A film producer, a musician and two music producers had filed suit against Warner/Chappell Music in 2014 after they paid fees ranging from a few hundred dollars to a few thousand to use "Happy Birthday" in musical productions.

"Happy Birthday" was written in 1893 by two sisters: Mildred and Patty Hill. Warner/Chappell Music claimed that they obtained the copyright to the song from a company that had gotten it from the Hill sisters' publisher, Clayton Summy. The plaintiffs had argued that the copyright covered only specific piano arrangements for the song, not the song and lyrics themselves.

The song will officially become part of the public domain on the case's final settlement date which is set sometime by the end of March.