Implement a code to solve Ux = b for x

I am supposed to write a code that will solve Ux = b where U,x and b are matrices. Using the built in MATLAB commands for this (i.e. x = U\b) is not acceptable. This is the code I have that I wrote from a general outline from my TA:

function [ x ] = utrisolve( A,b )
%UNTITLED6 Summary of this function goes here
%   Detailed explanation goes here
A;
b;
[n1,n2] = size(A);
if n1 ~= n2
    print('matrix must be square');
    return;
end
    AA = A;
    bb = b;
    while n2 ~= 0
        x(n2) = bb(n2)\AA(n1,n2);
        v = AA(1:n1-1,n2);
        AA = AA(1:n1-1,1:n2-1);
        bb = bb(1:n2-1)-v.*x(n2);
        n2 = n2-1;
        n1 = n1-1;
    end

end

and when I enter the following into the command window I get the following error and do not know how to fix it:

>> A = [-1 1 2 0; 0 2 -1 2; 0 0 -3 -1; 0 0 0 4];
b = [1; 1; 1; 1];
[x] = utrisolve(A,b)
Error using  -
Matrix dimensions must agree.

Error in utrisolve (line 17)
        bb = bb(1:n2-1)-v.*x(n2);

Help Please!

if true
  % code
end