The internal error message was: "Id not found"

[jarble]

MiniZinc 2.8.2 produces an error message when I compile and run this program using the Chuffed 0.13.1 solver:

var int: a;

constraint a = max(a,6);
constraint a = max(a,3);
constraint a = max(a,1);

solve satisfy;

output [show(a)];

Why does MiniZinc produce this error?

MiniZinc has encountered an internal error. This is a bug.
Please file a bug report using the MiniZinc bug tracker.
The internal error message was: 
"Id  not found"

The error doesn't occur when I run the following program, which should be equivalent to the above program:

var int: a;

constraint a >= 6;
constraint a >= 3;
constraint a >= 1;

solve satisfy;

%the output is 6
output [show(a)];