My shark following script only works the first time, whats wrong?

So everything works fine the first time, the shark follows me and kills me. But the second time it says that the torso on Line 6-7 is nil and the shark just freezes in place :(

local bodypos = Instance.new("BodyPosition", script.Parent)
while true do
    wait()
    for i, v in pairs(game.Workspace:GetChildren())do
        if v:IsA("Model") and v:FindFirstChild("Humanoid") then
        local torso = v:FindFirstChild("Torso")
        magnitude = (torso.Position - script.Parent.Position).magnitude
        if magnitude < 500 then
            --script.Parent.Rotation = torso.Rotation
            bodypos.MaxForce = Vector3.new(math.huge, math.huge, math.huge)
            bodypos.Position = torso.Position
            --bodypos.P = 10000
            --bodypos.D = 100000
            end
        end
    end
end

Help will be really appreciated :>