Studio 60 on the Sunset Strip actress Amanda Peet, who is set to marry Troy screenwriter David Benioff this weekend, is having some pre-wedding jitters, she says.

In an appearance Wednesday night on the Late Show with David Letterman, Peet admitted, "I think I've been pretty mellow, but recently I've been having recurring nightmares about, not the ceremony, but the party. They're sort of adolescent fears of just being lame, having a lame party."

Asked to describe her nightmares, the actress told Letterman, "I go to the bathroom and I come back to the party and everyone is gone except for a few distant uncles of David's left."

Peet, who is expecting her first child with Benioff, said she's four months pregnant and that her growing figure has earned her the nickname "Fatty" on the Studio 60 set.

"See, that's not right," answered Letterman. "That's just not right."

But Peet doesn't seem to mind: "I think it's really funny," she replied.

She also said she and Benioff don't know the gender of their baby, although she added, "I do want to know."

Peet, 34, and Benioff, 36, announced their engagement in July 2005. They've been dating for four years, Peet told Letterman, and met when "a mutual friend, Peter Blake, fixed us up on a blind date."

Of their early courtship, Peet said, "I thought he was a little cocky. … He made fun of my thumbs."

"What's wrong with them?" asked Letterman.

"Don't zoom in, please!" Peet said, as she showed the talk-show host her thumb. "My dad has those hammer thumbs. The nail is a sliver and the rest is a big, fat thumb."

When Letterman told her, "There is nothing wrong with your thumb," Peet joked, "Well, maybe I should have married you!"