An administrative error caused Ajax to leave club-record signing Sebastien Haller off of their Europa League squad, manager Erik ten Hag confirmed Thursday.

“It’s a mistake that should not have happened,” Ten Hag said, as reported by The Associated Press.

Ten Hag said Haller’s name wasn’t checked off on the list Ajax submitted to UEFA.

“Never a dull moment in Amsterdam,” the 51-year-old added. “Of course, he is unbelievably disappointed.”

It’s unclear if UEFA will allow the club to rectify the error.

Ajax signed Haller from West Ham United during the January transfer window, paying €22.5 million for the French striker. The fee is also the most-ever spent on an incoming transfer by a Dutch club.

Haller has made an immediate impact as well. He’s scored twice in his first six appearances for Ajax and has yet to lose a game.

His time at West Ham was far less exciting. Signed from Eintracht Frankfurt in 2019 for €50 million, Haller failed to live up to the billing, recording a modest 10 goals in 48 Premier League appearances.