Numerical Problem on DTFT using MATLAB

CODE:

w=0:0.01:2*pi;
r=0.9;
w0=pi/2;
a=-r*cos(w0);
alfa=2*a;
beta=r^2;
num=[1 a];
den=[1 alfa beta];
h=freqz(num,den,w);
subplot(2,1,1);
plot(w/pi,abs(h));
ylabel('Magnitude');
subplot(2,1,2);
plot(w/pi,angle(h));

ylabel('Phase');