Remove creature on area

[MiPo91]

Hello, could someone tell me how to make this check if the creature is summon or not, because this scripts removes all the monsters in the specific area, but if they are not found it gives alot of errors inside the console...

for x = from.x, to.x do
	for y = from.y, to.y do
		for z = from.z, to.z do
			local v = getTopCreature({x=x, y=y, z=z}).uid
			if isPlayer(v) then
				selfSay("Someone is already killing boss, please wait until he's done!", cid)
				return
			elseif isMonster(v) then
				table.insert(monsters, v)
			end
		end
	end
end

for i = 1, #monsters do
	doRemoveCreature(monsters[i])
end

the error

[20:10:04.100] [Error - NpcScript Interface]
[20:10:04.101] data/npc/scripts/killinginthenameof.lua:onCreatureSay
[20:10:04.101] Description:
[20:10:04.101] (luaDoRemoveCreature) Creature not found