If you’re going to be an ass in dealing with a visiting team … perhaps you first should make sure your case is watertight before you launch on your taunting.

A Mets fan, maybe 14, 15 years old, reached over the railing in left field to catch a long fly ball hit by Mets infielder Wilmer Flores. With a glove he presumably brought along for the purpose.

Then it got annoying.

The kid waved the globe back and forth, giving Colorado Rockies outfielder David Dahl, a sort of nyaa-nyaa treatment for the Rockies rookie.

Then it got worse.

The camera went back to the kid, as seen in the video linked above, and it showed him doing the Dikembe Mutumbo finger wave — the left-to-right motion that says, “No you don’t!”

Except Mutombo, the former NBA star, usually used the finger wave after blocking a shot. After he had actually accomplished something.

The kid? He was quite pleased with himself, as Flores circled the bases in what apparently had cut the Mets ninth-inning deficit to 7-3.

But Colorado’s manager Walt Weiss asked for the play to be reviewed. His contention was that the kid had violated the rules by bending over into the playing area — perhaps taking the ball just ahead of Dahl catching it. On replays, it did not appear the ball would clear the fence, or even touch it.

Umpires ruled in the Rockies favor, and Flores’s home run came off the scoreboard. The play officially became a fly out to left field — because of fan interference.

The Mets were not happy.

Flores later said umpires had assumed Dahl was going to make a catch, except with a bit more emphasis than that. It was possible to sympathize with him, because Wilmer Flores is not much of a hitter, and he lost what he thought were his 11th home run and 29th RBI.

Mets manager Terry Collins came onto the field, after the “out” sign was given, and made the same point, perhaps with the same language, and was ejected from the game.

And the little knucklehead in left field?

The kid seemed to be part of a group in some special seating, a row just behind the fence. He probably plays ball himself because he seemed comfortable enough with his glove that he made the illicit catch with no problem.

(And he is just young enough, barely, to go to a game with a glove and not be an object of derision. Adults catch balls bare-handed.)

He seemed smug about what had happened, but he may have cost Flores an extra-base hit. The ball, on the replay, doesn’t look quite deep enough to be a home run. But if Dahl had not caught it, it might have been a double for Flores.

So kids, before you snatch a ball from the field of play … and then taunt the opposition … make sure you know the rules of the game.