1D Poisson Solver Warning: Matrix is singular to working precision

I can't not see any problem with the matrix but can't fix the problem.

     %
x_min = 0;                         % min(domain)
x_max = 1;                         % max(domain)
H=10;
h=(x_max-x_min)/H;
x1 =(x_min:h:x_max)';
ureal=@(x) -x.^3+7;
ua = ureal(x_min);                % Dirichlet condition at x=x_min
ub = ureal(x_max); 
f=@(x) 6.*x;
n = length(x1);
b = -f(x1);
A = 2 * eye(n);
A = A + diag(-1*ones(n-1,1), 1) + diag(-1*ones(n-1,1), -1);
A = (1/h) * A;
A(1,:) = 0;     A(1,1) = 0;        
A(end,:) = 0;   A(end,end) = 0; 
b(1)=ua;
b(end)=ub;
u = A\b;
%%Plotting results
figure(1);
plot(x1,ureal(x1),'or');
hold on
plot(x1,u,'--.b');
xlabel('x');
ylabel('u');
legend('Exact Solution','Approximate Solution');
axis square tight; box on; grid on;

Thanks for the help:)