Last week, a contributor to a hard-line website posted a threat of violence against Letterman because the comedian had made fun of Al-Qaeda leaders on a previous show. Letterman, back from vacation Monday, played the threat for monologue laughs.

During Wednesday’s taping, Letterman asked Kutcher if his bowtie was bullet-proof. The actor replied that “you can’t be too careful” sitting next to a guy facing a threat.

Kutcher joined CBS’ “Two and a Half Men” after Charlie Sheen was fired. He and Letterman predicted success for the revamped sitcom.