Voronoi script not working

I'm very new to ghPython. I downloaded a simple ghPython script that uses the Voronoi command which worked on my machine. Then I tried to reproduce the results on my own and it doesn't work. I've got to the point where I think both programs are exactly the same as far as I can tell, but one of them produces nulls and the other works fine. Can some explain to me what's going on? 

Thank you!