Bessel-filter problems

[Bernd Heinze]

Hello,

I'm trying to use a bessel-filter to filter out frequencies out of a signal, 
but I keep on getting an error:

fNyq = 2.5e9;
fLow = 29e6;
fHigh = 33e6;

[b,a] = besself(10, [fLow/fNyq, fHigh/fNyq], 'z', 'stop');

error: bilinear: must have at least as many poles as zeroes in s-plane

This error also occurs if I decrease the filter order to 2.
Does anybody know how to resolve this error? Is there a way filtering a signal 
using the s-plane, as filter needs z-plane. Also is there some kind of 
"buttord" for the bessil filter computations?

Thanks for your help!
Bernd