impz command from signal package

[Robert Carroll]

Hi,

I’m running Octave-gui v. 4.0.3 on a mac running OS X High Sierra 10.13.6.

I am trying to call the impz command from the signal package. If I run 'pkg 
list’ I see that I have the package installed.

The command window displays ‘signal * 1.3.2 /Users/rob/octave/signal-1.3.2’.

However, I get error ‘incorrect call to roots’ if I try to call:

pkg load signal;
[z, fs] = impz (x, y);

Both are lines in my .m file. What is the correct way to access this function 
from the signal package?

thanks

rob