Domain not checked out, please see inside...

[parisse]

Actually, -3 is part of the domain because a CAS considers rational fraction as equivalent to their irreducible representation. But it should not be part of the answer (maybe because solving a/b>0 is equivalent to a*b>0, I'll have a look). But anyway CAS are not implemented to solve particular cases (that would be too costly), you should not be surprised to see things like that. In other words, answers are expected to be generically correct.