Hi, 
So I am trying to use Maple to find the integer points in a region(s) bounded buy a set of inequalities. But for a specific set of inequalities, in four variables,
inequalities:= [-a1-a2+a3+a4 <= 2, a1+a2 <= 3, -2*a1 <= -1, -2*a2 <= -1, -2*a3 <= -1, -2*a4 <= -1, -a1+a2+a3-a4 <= 4, -a1+a2-a3+a4 <= 4, a1-a2+a3-a4 <= 4, a1-a2-a3+a4 <= 4]; in four variables, vars :=[a1,a2,a3,a4];

I am getting the following error
Error, (in solver) invalid input: LinearMultivariateSystemInternal expects its 1st argument, eqns, to be of type ({list, set})({`<`, `<=`, `=`}), but received {a1-a2 = 0, -11/2+a1 <> 0}

Also, upon checking the inequalities by hand, I am sure that the region mentioned in the error doesnt come up. as based on the inequalities, it is not possible to have -11/2+a1 > 0. But though I could be wrong about my manual inequality analysis, the error is still suprising and worrying. 
Another point to note is that if I do a variable change, as
t1 := a1-a2;
t2 := a1+a2;
r1 := a3-a4;
r2 := a3+a4;
Then with new variables the inequalities are solved without any errors.

I am working with Maple2018 for this problem. 

Any help or insight into the root of this problem would be a lot of help.