Biden, speaking at the National Action Network’s annual breakfast, noted it was the birthday of King’s daughter-in-law, Arndrea Waters King.

“My wife has a rule in her family,” Biden said. “When it’s somebody’s birthday, you sing ‘Happy Birthday.’”

Then, he proceeded to lead the crowd in the song ― only for him to stumble when it came to her name.

“Wow!” Colbert said. “People are accusing him of forgetting her name. That’s not fair: He clearly never knew her name.”

See more in his Monday night monologue: