Lua Attempt to index local 'killer_' (a nil value)

[MorganaSacani]

I'm having this problem when I summon some monster to help me kill the algorithm's creature.
This is the problem that occurs:

timerEvent: []
callbackId:[]
function: []
error [...br-global\scripts\creaturescripts\monster\mad_knight.lua:66: attempt to index local 'killer_' (a nil value)
stack traceback:
[C]: in function '__index'
...br-global\scripts\creaturescripts\monster\mad_knight.lua:66: in function <...br-global\scripts\creaturescripts\monster\mad_knight.lua:57>]

I will warn you that this is an onDeath function
And here is a part of the algorithm:

for cid, damage in pairs(creature:getDamageMap()) do
        -- The 'killer_' variable is for returning all characters involved in the creature's death.
        local killer_ = Creature(cid)
        if killer_:isPlayer() then

Line 66:

if killer_:isPlayer() then