A computer glitch has allowed all of American Airlines’ pilots to take vacation during the week of Christmas, according to ABC and Reuters. The error could leave thousands of planes grounded during one of the busiest travel weeks of the year.

Citing the union that represents pilots, Reuters reports that over 15,000 planes currently lack pilot assignments in December. “On Friday, management disclosed a failure within the pilot schedule bidding system,” the Allied Pilots Association said in a statement. “As a result, thousands of flights currently do not have pilots assigned to fly them during the upcoming critical holiday period.”

“Basically there’s a crisis at American for manning the cockpits.”

The glitch apparently originated in the system that bids for pilots’ time off based on seniority, leading to the shortage. “Basically there’s a crisis at American for manning the cockpits,” Dennis Tajer, a spokesman for the Allied Pilots Association, told Reuters.

BREAKING: American Airlines says computer glitch allowed all pilots to take vacation over Christmas week. Now union says thousands of flights are in jeopardy of cancellation. Americans says they’re offering 1.5x pay for pilots to fill in and expect to resolve the issue. — Sam Sweeney (@SweeneyABC) November 29, 2017

American says it’s working on a solution. One possibility is to entice pilots to work by promising extra pay. The airline sent an email to employees offering one-and-a-half times their normal rates to help fill in, but a grievance filed by the union against management said that the restrictions on overtime pay were a violation of the group’s contract.

The union claims that management at American has not reached out to devise a solution. “Because management unilaterally created their solution in violation of the contract, neither APA nor the contract can guarantee the promised payment of the premium being offered,” the union said.

Update November 29th, 3:13PM ET: A spokesperson for American Airlines said in a statement, “We are working diligently to address the issue and expect to avoid cancellations this holiday season. We have reserve pilots to help cover flying in December, and we are paying pilots who pick up certain open trips 150 percent of their hourly rate – as much as we are allowed to pay them per the contract. We will work with the APA to take care of our pilots and ensure we get our customers to where they need to go over the holidays.”