Is masturbating injurious?

I went to the doctors for a check-up the other day. It all seemed fairly routine to begin with, but after a couple of minutes, the doctor looked at me with a serious expression on his face.

“I’m sorry Sir, but I’m afraid I’m going to have to ask you to stop masturbating.”

“Why?”, I said, somewhat shocked. “Is there some kind of injury?”

“No, it’s not that at all, Sir. It’s just…” he paused, and looked down at me again. “Well, it’s really putting me off…!”