Polynomial Stability Test output

I'm trying to check the stability of a discrete system using Polynomial Stability Test (PST) block. The transfer function is H(z)=1/(1-az^-1). I have tried importing the vector u = [1 -a] from workspace, using that as an input to PST, and finally exporting the result back to workspace (to variable "simout"). Problem is, the result is always the same no matter what value "a" has: simout.signals.values is "<1x1x11 logical>" and contains only 1:s.

y = all(abs(roots(u)) < 1) works fine and gives correct results (=the system is stable when abs(a) < 1), but I need to do the same using Simulink. How can I get the correct results from PST?