keep 'em on the court

[ABC Coach]

Had a game today where I was the coach and not the ref. While one of my players was shooting a free throw, I called another player over to our bench area to give him some advice. This player stepped out of bounds to hear what I had to say. I told him to stay on the court, which the ref overheard. Immediately the ref turned and called a technical foul because my player wasn't on the court during the free throw.

I've never heard of this being any kind of violation, let alone a technical foul. Did he make the right call? If so, if anyone knows where I can find that in the FIBA rules, I'd love to see it.

Follow up question:
The ref allowed my player to shoot his free throws and then allowed the other team to shoot the technical and take possession of the ball for a throw-in. Was the ref correct is allowing us to finish shooting our free throws, or should we have lost that opportunity due to the technical.

I'm sorry this question is so drawn out. Any help is greatly appreciated.