Sunking D3 purpose

[m-Kresol]

I'm wondering what the purpose of D3 (1N4742=12V Zener diode) in the sunking is. I've also seen it on another schematic of a Centaur. First I thought polarity protection but this is done by the FET in the sunking, isn't it?
A though just hit me: could it be for overvoltage protection in case someone feeds 18V into it? doubling the 18V to 36 with the charge pump would most likely fry the opamps... If so, wouldn't it be better to use a 9.1V Zener?
Enlightenment would be cool. Thanks!

EDIT: scratch that. Google was my friend redirecting me to an old thread: http://www.madbeanpedals.com/forum/index.php?topic=685.0

In short: polarity and overvoltage protection for the charge pump. 9.1V would work too