Matlab - Subscript indices must either be real positive integers or logicals

%% runge kutta dos ecuaciones

x1=1

y1=0

h=0.05

t(1)=0;x(1)=x1;y(1)=y1

X=y

Y=0.1*(1-x^2)*y-x

for i=1

    t(i+1)=t(1)+i*h

    k(1)=h*x(t(i),x(i),y(i));

    l(1)=h*y(t(i),x(i).y(i))

    k(2)=h*x(t(i)+h/2,x(i)+k(1)/2,y(i)+l(1)/2);

    l(2)=h*y(t(i)+h/2,x(i)+k(1)/2,y(i)+l(1)/2);

    k(3)=h*x(t(i)+h/2,x(i)+k(2)/2,y(i)+l(2)/2);

    l(3)=h*y(t(i)+h/2,x(i)+k(2)/2,y(i)+l(2)/2);

    k(4)=h*x(t(i)+h,x(i)+k(3),y(i)+l(3));

    l(4)=h*y(t(i)+h,x(i)+k(3),y(i)+l(3));

    w(i+1)=x(i)+(1/6)*(k(1)+x*k(2)+x*k(3)+k(4));

    z(i+1)=y(i)+(1/6)*(l(1)+2*l(2)+2*l(3)+l(4));

end

plot(t,w,'r-',t,z,'b-')

x(1)=x1   % aca x se comporta como un vector

x(t(i),x(i),y(i))   % aca se comporta como una función.