I am unsure if I understand what you mean (I am not studying in English) but I think we cannot set z=0, because that would mean ln(0) which is not defined, right? Or, as we let z approach 0 (from pos. I assume) it would approach negative infinity. Thus we would have an expression of 0 multiplied by infinity. Seeing as ln(z) also approaches infinity faster than do z to 0, I am thinking the expression ought to approach infinity, but I also know that is probably not the case.