Operand following / has invalid type

Hey,

I'm trying to make a program that populates a binary matrix keeping as 
many blank lines as possible, e.g.

|00|
|11|
|10|

instead of

|10|
|10|
|10|


Since I know the sum of the lines won't be too big I decided to use an 
approach based on division, but I'm getting a "operand following / has 
invalid type" error.

/* binary matrix */
var m{x in X, y in Y} binary;
/* minimizes total (sum of line)^-1, a full line is better than two 
half-full lines*/
minimize maxEmpty: sum{x in X} 1/(1 + sum{y in Y} m[x,y]);

I made a few tests and some constraints, I'm pretty sure it's not a 
problem with the sets, the matrix, or a typo.
If I had to guess I'd say the error is being caused by the nested sums, 
but I couldn't find much on that.

Can this be fixed in any way?
Thanks.