United Kingdom, April 24 -- Penn Badgley found Michelle Trachtenberg's death "surreal".

The 38-year-old actor's former 'Gossip Girl' co-star passed away in February at the age of 39 as a result of complications from diabetes, and though they hadn't seen one another "in over a decade", Penn was still very upset by the news.

He told E! News: "I can't claim to have known her well but it's surreal. Really surreal, really saddening."

The 'You' actor - who is expecting twins with wife Domino Kirke and is also dad to James, four, and stepdad to her 16-year-old son Cassius - paid tribute to Michelle's "really pure heart".

He added: "What I remember of her is she was incredibly quick to laugh. She was really joyous. We all have our ups and dow...