WASHINGTON — A rookie mistake burned Francisco Alvarez and the Mets at an inopportune time.

The young catcher got picked off first base — with the bases loaded and Brandon Nimmo at the plate — to extinguish a threat in Monday’s second inning at Nationals Park.

Alvarez, the No. 9 batter, had drawn a walk against Patrick Corbin to set up Nimmo with a chance at cracking open a game that was tied at 1.

But two pitches into the at-bat, opposing catcher Keibert Ruiz noticed Alvarez had danced too far off first base.

Ruiz caught a ball from Corbin and gunned to former Met Dominic Smith, who tagged Alvarez’s hands before they could reach the bag.

The 21-year-old catcher stayed on the dirt for a moment aware of the mistake he had made before heading to the dugout.

In ways beyond baserunning, though, Alvarez has mostly impressed in his first extended time in the big leagues with both Omar Narvaez and Tomas Nido on the injured list.

Alvarez has handled a veteran pitching staff, been better than advertised at framing pitches and occasionally flexed his power, with three home runs in his first 23 games.