“Hey Arnold!” writer and voice actor Steve Viksten died Monday at the age of 54.

Viksten voiced the character Oskar Kokoshka, a mooch with an ambiguous Eastern European accent, on the iconic Nickelodeon TV show. There’s no word yet as to the cause of his death.

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In addition to his memorable performance as Oskar, Viksten shined as a writer. According to The A.V. Club, Viksten wrote the “Hey Arnold!” movie, as well as some fan-favorite episodes including “Arnold’s Hat” and “Stoop Kid.” His writing credits also include episodes of “Rugrats,” “Recess,” “Duckman” and “The Simpsons.”

“We are deeply saddened to learn that Steve Viksten has passed away. Steve was a gifted artist and we were honored to have the opportunity to work with him on our beloved shows ‘Hey Arnold!’ and ‘Rugrats.’ Our heartfelt condolences go out to his loved ones,” Nickelodeon, who confirmed news of his death, said in a statement to The Huffington Post.

Craig Bartlett, who created “Hey Arnold!”, posted a memorial about Viksten to his Facebook page on June 24.

“Steve Viksten, who wrote many of my favorite Hey Arnold! Stories, died yesterday. I was lucky to be near to so much talent for so long. RIP,” the post reads.

Commenters have since been showering the thread with their condolences.

“RIP Steve Viksten… Thank you’ve for using your talent and allowing it to bring joy to thousands of kids, teens and adults,” wrote one.

“RIP, thanks for contributing greatness to my childhood,” wrote another.