Weird results for the solution of a simple equation

[arvin84@…]

OM ver.: 539229e

for the simple following model:

model a 
  Real x1; 
  Real x2; 
equation 
  x1 + x2 = 0; 
  x1 - x2 = 1; 
end a;

i get the following wrong answer:

x1=0.5;
x2=0;

[arvin84@…]

when i modify the code to the following:

model a 
parameter Real x1(fixed=false); 
parameter Real x2(fixed=false); 
initial equation 
x1 + x2 = 0; 
x1 - x2 = 1; 
end a;

i get the correct answer, x1=0.5, x2=-0.5;