i have a problem with flanger code?

here is my code: function y = flanger1(x,Fs)

F=1; d=1; a=0.7; index=1:length(x); D=d/2*(1-cos(2*pi*(F/Fs)*index));

for n=3:length(x), y(n) = x(n)+a*x(n-D(n)); end; end

i try to do some flange effect and i have a problem with the for loop it say i have a error but i dont know how to solve it does anyone have an idea?

thanks