A man used a vulgar insult for the president when calling into a Christmas event on Friday and speaking with President Joe Biden and first lady Jill Biden.

The Bidens had been speaking with children and their parents who had dialed into a phone line run by the North American Aerospace Defense Command, or NORAD, which tracks Santa Claus’ path across the globe. The president and first lady surprised callers, asking the children what they wanted for Christmas and wishing them a happy holiday.

A father who identified himself as “Jared” spoke with the Bidens about his children, then, as the call ended, said, “Let’s go, Brandon.”

The phrase is a vulgar insult that has become code for the phrase, “F— Joe Biden.” It stems from a reporter misinterpreting a chant after a NASCAR race and is popular among conservatives opposed to the president.

Biden responded by repeating the phrase. “Let’s go, Brandon,” he said. “I agree.”

The first couple then moved on to the next callers.

Operators do not screen participants for political affiliation before connecting them with the president, who sometimes will surprise callers on Christmas Eve, according to a NORAD official.

As part of its “Santa-tracking,” NORAD takes calls from many people, mostly children who want to find out when Santa will visit their house, the official said. Most years the first lady and perhaps the president take some of these calls, and NORAD does not publicly announce when the first family will be participating, according to the official.

When a child calls in during such a time, an operator usually asks to talk to a parent to be sure they are OK with their child talking to the president — but they do not ask political questions of the parent or otherwise try to screen them by political affiliation, the official said.

They do not ask since it is not a political event and because the vast majority of callers are not the type to be on the “naughty list,” the official said.

The White House did not respond to a request for comment about the exchange.

Earlier in the day, the president and first lady visited patients and families at Children’s National Hospital in Washington, and visited an outdoor Christmas tree in a neighborhood near the White House.

The first family plans to spend Christmas at the White House.