Upgrade spell issue

[MadMOOK]

data/spells/scripts/attack/berserk.lua:13: attempt to perform ari thmetic on local 'attack' (a nil value)

local SPELL_LVL_STORAGE = 18479

local combat = createCombatObject()
setCombatParam(combat, COMBAT_PARAM_TYPE, COMBAT_PHYSICALDAMAGE)
setCombatParam(combat, COMBAT_PARAM_EFFECT, CONST_ME_HITAREA)
setCombatParam(combat, COMBAT_PARAM_USECHARGES, true)

local area = createCombatArea(AREA_SQUARE1X1)
setCombatArea(combat, area)

function onGetFormulaValues(cid, level, skill, attack, element, factor)
local combat_formulas = {  
[0] = {min = (skill + attack*1) +level/5 -10, max = (skill + attack*2) +level/5 -20},  
[1] = {min = (skill + attack*2) +level/5 +30, max = (skill + attack*2) +level/5 +40},  
[2] = {min = (skill + attack*2) +level/5 +80, max = (skill + attack*3) +level/5 +130},  
[3] = {min = (skill + attack*2) +level/5 +150, max = (skill + attack*3) +level/5 +250} 
} 

local lvl = getPlayerStorageValue(cid, SPELL_LVL_STORAGE) 

lvl = lvl == -1 and 0 or lvl
x = combat_formulas[lvl]
return -x.min, -x.max
end

setCombatCallback(combat, CALLBACK_PARAM_LEVELMAGICVALUE, "onGetFormulaValues")

function onCastSpell(cid, var) 
return doCombat(cid, combat, var)
end