please help about butterworth filter

if true
  % code
endhi

bandpass filter function returns NaN in some samples:

[a,b]=butter(4,[8 30]/(125/2) ,'bandpass'); filtered_signals=filter(a,b,x);

x: 5000 rows and 2 columns

I solved that by filtering the data in blocks(32 samples). the result was not the same but when i increased the samples to 320 i got the same result. Is it true to solve NaN problem.