ODE45 in feedback loop

Hello!

I'm trying to plot a sum of differential equation system solutions using ODE45. Problem is my code doesnt wanna run more than 41 times.

%   SLAVE SYSTEM
S=1;       
xs = [];
for i=1:1:42
    f_s = @(t,x_s) [x_s(2)+k1*S;
        x_s(3)+k2*S;
        a1*x_s(1)+a2*x_s(2)+a3*x_s(3)+10^(-4)*(exp(x_s(1)/0.026)-1)+k3*S];
    [t,xs] = ode45(f_s,[0 0.001],[0.1 0.1 0.1]);
    x_slave_summa = xs(:,1)+xs(:,2)+xs(:,3);
    figure(4)
    plot(t,x_slave_summa)
    S = c1*(xs(i,1)-xg(i,1))+c2*(xs(i,2)-xg(i,2))+c3*(xs(i,3)-xg(i,3));
    i=i+1;
end

Idea is basically that I wanna solve equation system where f(y1,y2,y2) is as written in my code and I calculate S from previous diff eq solutions last sample (S=1 -> solve diff eq system -> calculate new S -> S=new_S ->solve diff eq system ......). If I increment for loops end value I get:

Index in position 1 exceeds array bounds (must not exceed 41).

Error in test (line 76)

S =

c1*(xs(i,1)-xg(i,1))+c2*(xs(i,2)-xg(i,2))+c3*(xs(i,3)-xg(i,3));

What is causing this problem and how can I get my loop to run longer?

Thanks!