Fog of War

Hello! Amazing library. I just have one problem with it, whenever I try to make it so the fog only works on one area (Such as the first Z layer), and then I change to the second map, every step I take I get this error.

runtime error: Cannot execute null.value().
proc name: update (/FogClient/proc/update)
usr: Renparo (/mob)
src: /FogClient (/FogClient)
call stack:
/FogClient (/FogClient): update(/FogSight (/FogSight))
/FogSight (/FogSight): tick()
/FogSight (/FogSight): loop()
/FogSight (/FogSight): New(Renparo (/mob), /FogClient (/FogClient), 2)

Any chance I can get some insight?