Wimbledon organizers have apologized and explained that “human error” was the reason behind the electronic line-calling system being turned off during a match on Sunday.

John McEnroe clashed with fellow commentator Andrew Castle over Wimbledon’s line-call system during a live broadcast, following a controversial moment involving Anastasia Pavlyuchenkova.

Russia’s Anastasia Pavlyuchenkova has questioned the bias of the officials at Wimbledon after controversial calls during the fourth-round contest against Britain’s Sonay Kartal.