Why does this two-line change break this minizinc set-cover program?

The program below (adapted from http://www.hakank.org/minizinc/set_covering4b.mzn ) is a solution to the set-cover problem (example data provided at end of question). This runs correctly.

int: num_alternatives;
int: num_objects;
par set of int: ALTERNATIVES = 1..num_alternatives;

% costs for the alternatives
array[ALTERNATIVES] of int: costs; 

% objects covered by the alternatives
array[ALTERNATIVES] of var set of 1..num_objects: a;

% decision variable: which alternative to choose
array[ALTERNATIVES] of var bool: x; 

% the objective to minimize
var int: z = sum(i in 1..num_alternatives) (x[i]*costs[i]); 
solve minimize z;

constraint
   forall(j in 1..num_objects) (
     sum(i in 1..num_alternatives) (x[i] * bool2int(j in a[i])) >= 1
   )
;

output
[
  "x: " ++ show(x) ++ "\n" ++ 
  "a: " ++ show(a) ++ "\n"
];

However, if I replace the a definition above:

array[ALTERNATIVES] of var set of 1..num_objects: a;

with these two lines that seem to me to be equivalent:

var set of int: OBJECTS = 1..num_objects;  
array[ALTERNATIVES] of OBJECTS: a;

...suddenly I get the following error:

MiniZinc: type error: type-inst must be par set but is `var set of int'

This confuses me. What did I even change? In each case a is an array of sets of ints. The type-instance is a var set of int in each case, but the second one throws an error and the first one doesn't for some reason?

---

Here's some data which can be dropped in the bottom of the .mzn code file to produce a self-contained, runnable example:

% data
num_alternatives =  10;
costs = [ 19, 16, 18, 13, 15, 19, 15, 17, 16, 15];
num_objects = 8;

% the alternatives and the objects they contain
a = [
  {1,6},
  {2,6,8},
  {1,4,7},
  {2,3,5},
  {2,5},
  {2,3},
  {2,3,4},
  {4,5,8},
  {3,6,8},
  {1,6,7}
];

You could write it as follows:

int: num_alternatives;
int: num_objects;
set of int: ALTERNATIVES = 1..num_alternatives;
set of int: OBJECTS = 1..num_objects;

% costs for the alternatives
array[ALTERNATIVES] of int: costs; 

% objects covered by the alternatives
array[ALTERNATIVES] of var set of OBJECTS: a;

% decision variable: which alternative to choose
array[ALTERNATIVES] of var bool: x; 

% the objective to minimize
var int: z = sum(i in ALTERNATIVES) (x[i]*costs[i]); 
solve minimize z;

constraint
   forall(j in OBJECTS) (
     sum(i in ALTERNATIVES) (x[i] * (j in a[i])) >= 1
   )
;

output
[
  "x: " ++ show(x) ++ "\n" ++ 
  "a: " ++ show(a) ++ "\n"
];

In your experiment

var set of int: OBJECTS = 1..num_objects;  
array[ALTERNATIVES] of OBJECTS: a;

a is an array of integers in the range 1..num_objects. But you intended an array of sets of integers in that range.