[metapost] Metapost doesn't like `z01' for point names

[Rodolfo Medina]

Please excuse the basic question.
If I name a point `z01' Metapost complains:

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%file test.mp
beginfig(1);

z0 = (0,0);
z1 = (1,1);

z01 = 0.5[z0,z1];

endfig;

end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

$ mpost test
This is MetaPost, Version 0.641 (Web2C 7.4.5)
(test.mp
! Inconsistent equation (off by -0.5).
<to be read again>
                   ;
l.8 z01 = 0.5[z0,z1];

?


Instead, if I name it `z11' it's okay.  Why?  I really would like to name it
`z01': is it definitely impossible?

Thanks for any reply
Rodolfo