wxMaxima: function of complex variable

Hi
I'm trying to differentiate function of complex variable:
(%i1) z:zx+zy*%i;
(%o1) %i*zy+zx
(%i2) f(z):=z^2;
(%o2) f(z):=z^2
(%i3) diff(f(z),z,1);
Non-variable 2nd argument to diff:%i*zy+zx -- an error.
Quitting. To debug this try debugmode(true);
(%i4)


What is wrong ?

Adam
.