Contact on Jump Shooter

[Stripes33]

This situation happened last night during a varsity boys game.

Late 4th quarter on a fast break, guard swings the ball to the wing for an open 3 point shot. The defender runs to try and challenge the shot and pokes the shooter in the stomach.

I immediately blow my whistle as the ball falls short of the rim - shooting 3 free throws. The offending teams coach asks me what his player did and I explain that his player committed a foul by illegally contacting the shooter with the poke. He argues that the contact is not foul and should be ignored.

Keep in mind I hadn't seen this at any other point or shot attempt in the game other than this critical point.

Thoughts on this call?