Barcelona were penalized by Athletic Bilbao in a scoreless draw when they were six points clear in the league.
Lionel Messi had the best chance for Barca when his Lup hit the crossbar from outside the area.
The hosts were the better team and Barca keeper Marc-Andre ter Stegen made excellent saves to refuse Markel Susaeta and Inaki Williams.
Athletic ended with 10 men, as Oscar de Marcos showed a second yellow card for handball in added time.
The undefeated champions in eleven league games Barcelona missed the chance to stand eight points behind the runner-up Real Madrid, which was defeated 3-1 in the main derby on Saturday against Atletico.
It was the first time they had scored in a league game in 38 games, scoring since the goalless draw with Getafe on 11 February 2018.