Specify discrete transfer functions in DSP format

I use the function 'filt' to specify discrete transfer functions in DSP format for state space system .

   num=cfirpm(60,[-1 -0.5 -0.4 0.7  0.8 1],@lowpass);
   den=[1 0];
   TF= filt(num,den);

I found this warning : the numerator or denominator of this transfer function has complex-valued coefficients.

How can i solve this problem ?