why am i get getting this error "Input to ROOTS must not contain NaN or Inf " when i tried to design a LPF in MATLAB?

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pavan sunder 2016-1-27

I want to design a 2 nd order LPF with the following specs:

sampling frequency=200 Hz; cut off frequency 11Hz;

The transfer function is given as: H(z)=(1-2(z^-6)+(z^-12))/(1-2(z^-1)+(z^-2))

The matlab code i wrote is:

fs=200;T=1/fs;fnyquist=fs/2;

fc=11/fnyquist;

fstop=30/fnyquist;

N=2

d=fdesign.lowpass('Nb,Na,Fp,Fst',12,2,fc,fstop);

v=designmethods(d,'type');

opts=designopts(d,'iirlpnorm');

hd=design(d,'iirlpnorm','InitNum',[1 0 0 0 0 0 -2 0 0 0 0 0 1],'InitDen',[1 -2 1]);

freqz(hd);

when i tried to run the code i am getting the error "Input to ROOTS must not contain NaN or Inf ";

why am i getting this error. plz let me know what is the mistake in my code?

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