The actor, 36, has three children with ex-wife Sadie Frost, and became a Dad for the fourth time this September, when his ex-girlfriend Samantha Burke gave birth to their daughter, Sophia.
But little Sophia didn't cross his mind while chatting Thursday about his holiday plans. "I usually try to take the children on a bit of an adventure," Law told Letterman of his traditions. (This year, he says they're relaxing on a beach.)
When the late-night comedian followed up to ask "how many kids?" Law replies "three," and fails to correct himself.
When Burke, a 24 year-old aspiring actress, announced her pregnancy in July, Law initially disputed the paternity claims. The Oscar-nominated star soon came around, and his rep released a statement confirming that he was the father, and that he intended "to be a fully supportive part of the child's life."