Attempt to index nil with 'Position'?

I'm basically just making an enemy npc that goes to the nearest npc in the game and attack each other. This script is located in the npc. The error is in line 29. Help please:

local dis = math.huge
local rot = script.Parent.HumanoidRootPart.Position
local animation = script:WaitForChild("Animation")
local hold = script:WaitForChild("Hold")
local human = script.Parent:WaitForChild("Humanoid")
local ani = human:LoadAnimation(animation)
local anima3 = human:LoadAnimation(hold)
anima3:Play()
ani:Play()

function findNearestHumanoid(pos)
    local list = game.Workspace:GetChildren()
    for i = 1,#list do
        local v = list[i]
        if v.ClassName == "Model" and v ~= script.Parent then
            local rootpart = v:FindFirstChild("HumanoidRootPart")
            local humanoid = v:FindFirstChildWhichIsA("Humanoid")
            if humanoid ~= nil and rootpart and humanoid.Health <= 0 then
                if (rootpart.Position - pos).Magnitude < dis then 
                    local space = rootpart
                    return space
                end
            end
        end
    end
end

while true do
    wait(1)
    local map = findNearestHumanoid(rot).Position - rot  -- Attempt to index nil with 'Position' here
    local mag = map.Magnitude
    if mag < 3 then -- th NPC stops when they get close enough
        ani:Stop()
    else
        ani:Play()
    end
    local target = findNearestHumanoid(script.Parent.HumanoidRootPart.Position)
    if target ~= nil then
        script.Parent:FindFirstChildWhichIsA("Humanoid"):MoveTo(target.Position)
    end
end

Thank you!