Lua Zombie event

[Muumi]

Hey

when I start zombie event and people join the event after a while the event starts and gives me this error

[12/06/2016 03:22:29] [Error - GlobalEvent Interface] 
[12/06/2016 03:22:29] data/globalevents/scripts/custom/onthink.lua:onThink
[12/06/2016 03:22:29] Description: 
[12/06/2016 03:22:29] data/lib/zombie_event.lua:123: attempt to index local 'pir' (a number value)
[12/06/2016 03:22:29] stack traceback:
[12/06/2016 03:22:29]  data/lib/zombie_event.lua:123: in function 'spawnNewZombie'
[12/06/2016 03:22:29]  data/globalevents/scripts/custom/onthink.lua:6: in function <data/globalevents/scripts/custom/onthink.lua:1>
[12/06/2016 03:22:29] [Error - GlobalEvents::think] Couldn't execute event: ZombieGlobalThink

I'm using this script LINK
thanks