[Help-glpk] Invalid type error for abs() function

[Michael]

Hi,
   I'm getting the following error when checking my MathProg model:

test.mod:50: argument for abs has invalid type
Context:  ( abs ( map [ i , k ] * X [ k ] - map [ j , l ] * X [ l ] )
Model processing error

where map is defined as:
var map{i in MDFG, j in ADEPT}, binary;

and X is defined as:
param X{k in ADEPT};

and the indices i,j,k,l are dummy indices coming from 3 sum statements
like sum{i in MDFG, j in MDFG} sum{k in ADEPT} sum{l in ADEPT}

I haven't found anything in the doucmentation that would suggest to me
that this is an invalid statement but I'm obviously missing something.

Any help is greatly appreciated.

Thanks,
-Michael-