Argentinos Juniors have been comfortably the best side in the Nacional B this season but despite strolling to the league title found the luck of champions desert them on Wednesday evening, allowing Instituto to claim a slightly fortuitous Copa Argentina victory.

Gabriel Heinze’s side were far the better side but found themselves a goal down midway through the first half thanks to a soft penalty decision that provided Paulo Rosales the chance to open the scoring.

Shortly after Los Bichos thought they were level only for a linesman’s flag to see the scrappy equaliser ruled out, Francisco Fydriszewski adjudged to be interfering with play from an offside position as the ball bobbled over the line.

Instituto certainly got the rub of the green with the two critical decisions but Argentinos still saw plenty of the ball and created chances to find a way back into the tie.

However, the champions couldn’t muster an equaliser and in the closing stages had goalkeeper Federico Lanzillota hobble off with what looked like a serious knee injury.

Instituto will play either River Plate or Atlas in the last 32.