Oh, UnitAlive. But I don't think the script has any problem, anyway, have you tried to update jasshelper?
scope Spell
private struct Data
endstruct
private function Periodic takes nothing returns boolean
local Data d = KT_GetData()
//If you want to stop, return true
//Else, return false
endfunction
private function Act takes nothing returns nothing
local Data d = Data.create()
//Do stuffs.
call KT_Add(function Periodic, d, 15.)
endfunction
endscope