Dear Sungho,

We are using your nice spm-fnirs toolbox and we got the same error message as reported by Victor. We used the defaults bandstop filters 0.12 0.5 and 0.7 1.5. The data was recorded with 50Hz but we resampled it to 10 Hz. Below is the error message.
Apply a temporal filter to hemoglobin changes...
Error using filter_with_correction (line 51)
Calculated filter coefficients have poles on or outside the unit circle and will not be stable. Try a higher cutoff frequency or a different type/order of filter.
Error in filter_with_correction (line 51)
error('Calculated filter coefficients have poles on or outside the unit circle and will not be stable. Try a higher cutoff frequency or a different type/order of filter.');
Error in ft_preproc_bandstopfilter (line 283)
filt = filter_with_correction(B,A,dat,dir,usefftfilt);
Error in spm_fnirs_preproc (line 115)
for j = 1:nf, Y = ft_preproc_bandstopfilter(Y', P.fs, P.K.C.cutoff(j,:), forder, ftype, fdir)'; end

Any help will be welcome
Best
Ralf