[metapost] Strange error, bug ?

[Pétiard François]

Hello!

My computer'OS is Windows Vista Ultimate.
I work with MiKTeX 2.9 (MetaPost 1.504)

Here's my mp file:

prologues:=3;
outputtemplate:="%j.mps";
beginfig(1);
label(btex $-\infty$ etex,(0,0));
label(btex $\null-\infty$ etex,(50,0));
label(btex $-\,\infty$ etex,(100,0));
endfig;
end;

When I compile that file (mpost test.mp), the first label is not 
correct: there are two -
but the second and the third are correct.

The mpx file is:

% Written by metapost version 1.504
begingroup save _p,_r,_s,_n; picture _p; _p=nullpicture;
string _n[];
vardef _s(expr _t,_f,_m,_x,_y)(text _c)=
   addto _p also _t infont _f scaled _m shifted (_x,_y) _c; enddef;
_n0="cmsy10";
_s(char0&"1",_n0,1.00000,0.0000,0.0000,);
setbounds _p to (0,-0.8302)--(17.7114,-0.8302)--
  (17.7114,5.8115)--(0,5.8115)--cycle;
_p endgroup
mpxbreak
begingroup save _p,_r,_s,_n; picture _p; _p=nullpicture;
string _n[];
vardef _s(expr _t,_f,_m,_x,_y)(text _c)=
   addto _p also _t infont _f scaled _m shifted (_x,_y) _c; enddef;
_n0="cmsy10";
_s(char0,_n0,1.00000,2.2139,0.0000,);
_s("1",_n0,1.00000,12.1765,0.0000,);
setbounds _p to (0,-0.8302)--(22.1391,-0.8302)--
  (22.1391,5.8115)--(0,5.8115)--cycle;
_p endgroup
mpxbreak
begingroup save _p,_r,_s,_n; picture _p; _p=nullpicture;
string _n[];
vardef _s(expr _t,_f,_m,_x,_y)(text _c)=
   addto _p also _t infont _f scaled _m shifted (_x,_y) _c; enddef;
_n0="cmsy10";
_s(char0,_n0,1.00000,0.0000,0.0000,);
_s("1",_n0,1.00000,9.4091,0.0000,);
setbounds _p to (0,-0.8302)--(19.3718,-0.8302)--
  (19.3718,5.8115)--(0,5.8115)--cycle;
_p endgroup
mpxbreak

and the log is:

This is MetaPost, version 1.504 (MiKTeX 2.9)  24 MAR 2012 10:24
**test.mp
(mpost.mp (C:/Program Files/MiKTeX 2.9/metapost/base/plain.mp
Preloading the plain mem file, version 1.004)) (E:/TeX/Tests/test.mp 
[1{pdftex.map}<cmsy10.pfb>]
)
1 output file written: test.mps

I don't understand...

Any help is welcome!

Best regards

François