Attempt to index number with 'HumanoidRootPart' error?

I have a code for a tower defense game. Unfortunately. Line 6 in the code below is giving me the error. Below is the code. (yes, there is more code, but this is only the part thats causing trouble)

function FindNearestTarget(newTower)
    local maxDistance = 15
    local nearestTarget = nil
    for i, target in ipairs(workspace.Mobs:GetChildren()) do
        print(newTower.HumanoidRootPart.Position)
        local distance = (target.HumanoidRootPart.Position -newTower.HumanoidRootPart.Position).Magnitude
        --print(target.Name, distance)
        if distance < maxDistance then
            nearestTarget = target
            maxDistance = distance
        end
    end

    return nearestTarget
end