symbolic eig, problems with presentation and zero values

Please look at the following example

if true
clear all
A= sym([1 1 1; 1 1 1; 1 1 1])
% the eigenvalues of that matrix are l1=0 (alg mult=2) l2=3 (alg mult=1)
% the following matrix contains eigen vectors (not the same base as matlab uses)
V2=[-1 -1 1;2 -1 1; -1 2 1]
% Check
Z=A*V2(:,1)  % OK
Z=A*V2(:,2)  % OK
Z=A*V2(:,3)  % OK
B=inv(V2)*A*V2
z=B(1,3)
ps2=vpa(z,3)

so the value of B(1,3) should be zero, but since I have chosen symbolic variables matlab returns

   z=-18014398509481983/649037107316853453566312041152512
   z=-2.78e-17

that is a bit annoying, how can it be avoided?

Uwe Brauer