Implementing Z-Transform of -a^n*heaviside(-n-1)

Hello everyone, is it possible to implement a code that carries out the Z-Transform of an anticausal sequence like this -a^n*heaviside(-n-1)?

The code:

close all
clear all
syms a n z 
assume(a > 0)
assumeAlso(a < 1)
assumeAlso(a < z)
S = symsum((-a/z)^n*heaviside(-n-1), n, -Inf, Inf);
S = simplify(S, 'Steps',20)

but it does not work. The solution is z/(z-a).

Thank you for your time.