Trouble implementing crank nicolson scheme for 1D Diffusion Equation

Hi Folks, I am having issues coding up a piece of code I've written. It is the Diuffusion Equation with periodic boundary conditions using the crank nicolson scheme. I've given it a good amount of time and keep getting this error message:

Error using \ Matrix dimensions must agree.

Error in heat_cn (line 39) u(1:N-1,j+1) = A\RHS;

To be specific it has to do with solving the equation at each time step. Would really appreciate some help and guidance on this. Thanks!

function [u,err,x,t] = heat_cn(t_0,t_f,M,N)

% define the mesh in space
dx = 1/(N-1);
x = 0:dx:1;
x = x';

% define the mesh in time
dt = (t_f-t_0)/M;
t = t_0:dt:t_f;

% define the ratio r
r = dt/dx^2 

for i=1:N
  u(i,1) = sin(x(i));
end 
err(:,1) = u(:,1) - exp(-t(1))*cos(x);

A = zeros(N-1,N-1);
A(1,1) = 1+r;
A(1,2) = -r/2;
A(1,N-1) = -r/2;
for i=2:N-2
  A(i,i-1) = -r/2;
  A(i,i) = 1+r;
  A(i,i+1) = -r/2;
end
A(N-1,N-1) = 1+r;
A(N-1,N-2) = -r/2;
A(N-1,1) = -r/2;

for j=1:M
  RHS(1) = u(1,j) + r/2*(u(2,j)-2*u(1,j)+u(N-1,j));
  for i=2:N-1
    RHS(i) = u(i,j) + r/2*(u(i+1,j)-2*u(i,j)+u(i-1,j));
  end
  u(1:N-1,j+1) = A\RHS;  
  % impose periodicity
  u(N,j+1) = u(1,j+1);
  err(:,j+1) = u(:,j+1) - exp(-t(j+1))*cos(x);
end

plot(x,u(:,3));