Finite difference numerical method no flux boundary

Hi, I am trying to inplement a FTCS scheme which finds the concentration C at certain time tEnd. The boundary condition at w (end of y) have no flux. This is what i have come up with so far, but it is giving me an error. How do I fix this? Thanks!

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

Error in numerical2d (line 24)

C(i,n+1) = C(i,n)+sigma*(C(i-1,n)-2*C(i,n)+C(i+1,n));

y=linspace(0,w,ny+1); %ny=128
dy=y(2)-y(1);
dt= dy^2/(2*D);

nt=tEnd/dt+1; %calculate the number of timesteps required
sigma = (D*dt)/(dy^2);

C = zeros(length(ny),length(nt)); 

C(1,:)=C0; %boundary
C(:,1) =0; %initial

for n = 2:nt-1
    % Loop over internal points
    for i= 2:ny-1
        C(i,n+1) = C(i,n)+sigma*(C(i-1,n)-2*C(i,n)+C(i+1,n));
       
    end

     C(ny,n+1) = C(ny-1,n+1);
    
     %boundary condition at w
end

[darova]

I found a mistake

I believe that size of C should be

C = zeros(ny,nt)