Asked about his miss post-match, Choupo-Moting said: "Even for me it's hard to explain".
Victory would have given PSG their eighth Ligue 1 title after Lille earlier drew 1-1 at Stade de Reims but Sunday's draw left Thomas Tuchel's side on 81 points from 30 games.
The mind-boggling miss wound up costing PSG the game, having to settle for a 2-2 draw against Strasbourg, which represents the first time this season the Parisian heavyweights gave up points at their home stadium. In the 28th minute of a 1-1 game, Nkunku has a cross that was tipped and was spinning its way toward the back of the net for a goal, but Choupo-Moting made the freakish decision to try to play it.More news: Ethiopian report says faulty sensor data led to jet crash
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"I wondered if I was offside or not. I touched the ball, it stayed on the line and touched the post.I wasted this opportunity".
When everyone thought it was going to be 2-1, Choupo-Moting thought of stopping the ball on the goalline.
Goncalves's stunning volley from 18 metres put Strasbourg ahead seven minutes before the break against a PSG side who had previously been behind on the scoreline for only three minutes in Ligue 1 this season.
Nuno da Costa equalised for Strasbourg in the 26th minute before Thierry Laurey's side went into half-time with the lead thanks to Anthony Gonçalves.