Sub for shooter

[Clark Kent]

A1 is fouled and shooting two shots. A6 comes to the table (alone) to sub into the game. After the first ft the official blows his whistle and beckons him in, but he says he is subbing for the shooter. A1 makes his second shot and officials again blow their whistles and A6 then comes into the game, but instead of A1 going to the bench it was A3.

Didn't have my book on me (nor do I now) and I'm pretty sure there is no rule against it, but does anyone have a reference....or perhaps in this case a lack of a reference?

Thanks