Nina Dobrev took to her Twitter to wish her steamy ex boyfriend a happy birthday. Nina Dobrev even used a popular nickname for her "Vampire Diaries" co-star. But why couldn't Nina celebrate with her ex boyfriend on his 35th birthday?

"Happy Bday Smouldy! Hope you had a beautiful day filled w/ smiles+gluten free cake," Nina Dobrev tweeted to Ian Somerhalder. "But not too much cake,we're on the CW :P @iansomerhalder."

Ian tweeted a thank you to his fans for their birthday wishes, but didn't directly address Nina: "Thank you EVERYONE for all of these Birthday wishes-it's really special."

He also called the outpouring of love "really special," and took a moment to thank his mom "for pushing me out into this beautiful,magical&insane world to be challenged,loved and ALIVE..."

What a sweet son!

So why couldn't Nina Dobrev celebrate with her ex boyfriend on his big day? While some may think it would be weird to be there on an ex's birthday, fans of "The Vampire Diaries" will know that Nina and Ian are still good friends.

Unfortunately, Nina was busy eating at an In-N-Out-Burger on Sunday, Dec. 8 in a blue Elie Saab gown.

Dobrev, 24, was at yesterday's TrevorLive benefit for the LGBT youth hotline The Trevor Project at the Hollywood Palladium, E! News reports.

It was Dobrev's first time at the star-studded event; she attended as the guest of "Vampire Diaries" creator Kevin Williamson.

"The Trevor Project has helped so many people save lives and hopefully it will continue to save more," she said.

After meeting on the set of "Vampire Diaries," Nina and Ian dated for three years before breaking up earlier this summer. They play vampire girlfriend and boyfriend Elena and Damon on the CW series.