Script saying humanoid isn't part of model when it is?

I have some AI enemies that deal damage to me and I have to try and kill them, after I die they respawn. I can kill them before they respawn but after they respawn my sword script breaks. I get this error: "Zombie is not a valid member of Model" on Line 11. (Zombie is the humanoid inside my enemy, Dark,) I think this is because my respawn script removes the enemy before cloning it back into the world. In that time my sword script errors because there is no enemy. Although when the enemy gets cloned back my sword script still errors?

Enemy Cloning Script

z = script.Parent

backup = z:clone()

game:GetService('Players').PlayerAdded:connect(function(player)
    player.CharacterAdded:connect(function(character)
        character:WaitForChild("Humanoid").Died:connect(function()


        z:Remove()
        wait(2)
        backup.Parent = game.Workspace.Enemies
        backup.Head:MakeJoints()
        backup.Torso:MakeJoints()
        wait(15)
        z.Zombie.WalkSpeed = 16
end)
end)

    if z == nil then
        wait(4)
        backup.Parent = game.Workspace.Enemies
        backup.Head:MakeJoints()
        backup.Torso:MakeJoints()
    end
end)

Sword script

local enemies = game.Workspace.Enemies:GetChildren() --// All the children of the enemies folder.



for i, v in pairs(enemies) do
        if v:FindFirstChild("Zombie") then --// If the model has a humanoid
            wait(1)
            v.HumanoidRootPart.Touched:Connect(function()--// When the sword touches the npc
                script.Parent.Activated:Connect(function() --// If you click holding the sword it will damage.
                v.Zombie:TakeDamage(10) --// Npc takes 10 Damage

                wait(1)
            end)
            end)
            end
    end

I tried to add at the beginning of the sword script (didn't work)

while game.Workspace:FindFirstChild("Enemies") do

Thanks!