digital butterworth filter design error
I cannot run your code because I have not matlab on this pc, but I can try to give you some advice.

The first thing I will check is the way you defined the cut-off frequencies. The Matlab function uses normalized frequencies (look at the examples here http://it.mathworks.com/help/signal/ref/buttord.html), which means you have to define it in this way: lets assume you want to cut your signal at 5 Hz and your sampling frequency is $Fs$

cut_freq = 5;
Wn = cut_freq/(Fs/2);


Of course if you have a band-pass filter you have to define 2 cut-off frequencies and therefore $Wn$ will be a vector of 2 elements.

Hope this helps. As soon as I get my laptop I will try to run your code and see what happens.