Lerping Welded Part Causes It To Dissappear?

So i was lerping a part welded to the player's torso, but when i test it, the part dissapears, What i want to know is how can i make it so that the part stays welded or in a weld-like state and intact but still lerps

            local start = Poop.Size
            local finish = Poop.Size * Vector3.new(7,.44,7)

            for progress = 0, 1, 0.03 do

                 Poop.Size = start:lerp(finish, progress)
                wait()
            end 

If i were to try to fire the function a second time, there would be an error

Poop is not a valid member of Part