The film and TV director Lynn Shelton has died aged 54, due to a previously unidentified blood disorder. Shelton didn’t start directing feature films until she was 39, and was influential at the forefront of the mumblecore movement.

Directing films such as 2009’s Humpday and the mostly-improvised Your Sister’s Sister in 2011, the filmmaker also went on to direct episodes for a range of TV shows, including Mad Men, GLOW, New Girl, and the recently-released Little Fires Everywhere.

Her most recent film, Sword of Trust, starred Marc Maron, with whom Shelton had been in a relationship over the last year. “I am leveled, heartbroken and in complete shock and don't really know how to move forward in this moment,” says Maron in a statement shared by The Hollywood Reporter.

“She was a beautiful, kind, loving, charismatic artist. Her spirit was pure joy. She made me happy. I made her happy. We laughed a lot. We were starting a life together. I really can’t believe what is happening. This is a horrendous, sad loss.”

Mark Duplass, her fellow filmmaker and frequent collaborator, has also shared a tribute on social media, writing: “We lost our dear friend Lynn Shelton. We made so many things together. I wish we had made more.”

“Her boundless creative energy and infectious spirit were unrivaled. She made me better. We butted heads, made up, laughed, pushed each other. Like family. What a deep loss.”