Basic problem with differentiation and Maxima syntax

Hello,

I am trying to achieve the following:
Declare a function, differentiate it to obtain a second function and feed 0
as an argument to this function.

Here is what I did:

declare(%mu,constant);
f(%theta):=(1-%theta/%mu)^-1;
"f'"(%theta):=diff(%,%theta,1);
"f'"(%theta);
"f'"(0);

Here is what I got:
*************************
diff: second argument must be a variable; found 0
#0: f\'(%theta=0)
 -- an error. To debug this try: debugmode(true);
*************************

Any idea why this did not work? I just want to know the value of the
function when %theta=0...

Thanks in advance,

Julien.