Script classname not found

[tipforeveryone]

I attached this script to a Directional light

function Script:Start()
  sun = self.entity
end

Game started normally, but When I changed the changemapname variable (which can be found in Main.lua) to another map which also had the same Scripted Directional light. I recieved this "Script classname sun not found."

Is there something wrong with global variable ?

Edit: It seems my global variables were "stuck" when I change map. Is there anyway to RESET or RELEASE everything ? everytime I load a map, I want everything is clean. World:Clear() does not work well I think