Attempt to index nil with 'humanoid' (Roblox Studio)

Question

I'm trying to code a turret, shooting enemies when it gets in the range of the turret, and the code sometimes breaks giving me the error Script:31: attempt to index nil with 'Humanoid' This happens rarely and only does so when a zombie dies. Not only that but it seems some of the bullets created seem to make way more damage than they really should.

head = script.Parent.PrimaryPart

local function triggershoot(part)
    local beam = Instance.new("Part",script.Parent)

    beam.Anchored = true
    beam.CanCollide = false
    beam.Shape = "Block"
    beam.CFrame = head.CFrame
    beam.Size = Vector3.new(0.5,0.5,0.5)
    beam.Material = "Plastic"
    beam.BrickColor = BrickColor.new("Dark taupe")

    beam.CFrame = CFrame.lookAt(head.Position,part.Position)
    for i = 0, 50, 1 do
        wait()
        beam.CFrame = beam.CFrame + beam.CFrame.LookVector      
        beam.Touched:Connect(function(plr)
            if plr.Parent:HasTag("enemy") then
                plr.Parent.Humanoid:TakeDamage(3)
                beam:Destroy()
            end
        end)
    end
end

while true do
    wait()
    local parts = workspace:GetPartsInPart(script.Parent.zone)
    for _, part in pairs(parts) do
        if part:HasTag("enemy") and part.Parent.Humanoid.Health > 0 then

            local update = CFrame.lookAt(head.Position,part.Position)
            head.CFrame = update
            coroutine.wrap(triggershoot)(part)
            wait(1)
        end
    end
end

Answer 1

try moving the touch event function outside the forloop

head = script.Parent.PrimaryPart

local function triggershoot(part)
    local beam = Instance.new("Part",script.Parent)

    beam.Anchored = true
    beam.CanCollide = false
    beam.Shape = "Block"
    beam.CFrame = head.CFrame
    beam.Size = Vector3.new(0.5,0.5,0.5)
    beam.Material = "Plastic"
    beam.BrickColor = BrickColor.new("Dark taupe")

    beam.CFrame = CFrame.lookAt(head.Position,part.Position)
    beam.Touched:Connect(function(plr)
        local char=plr.Parent
        local hum=char and char:FindFirstChildOfClass"Humanoid"
        if char:HasTag("enemy") then
            hum:TakeDamage(3)
            beam:Destroy()
        end
    end)
    for i = 0, 50, 1 do
        wait()
        beam.CFrame = beam.CFrame + beam.CFrame.LookVector      
    end
    beam:Destroy()
end
while true do
    wait()
    local parts = workspace:GetPartsInPart(script.Parent.zone)
    for _, part in ipairs(parts) do
        local h=part.Parent;h=h and h:FindFirstChildOfClass"Humanoid"
        if part:HasTag"enemy"and h and h.Health > 0 then
            local update = CFrame.lookAt(head.Position,part.Position)
            head.CFrame = update
            coroutine.wrap(triggershoot)(part)
            wait(1)
            break
        end
    end
end