Problem with a sphere welded to the character?

I have a character spawning inside a sphere. When I turn CanCollide to false the character's legs keep going through the baseplate and then coming back up, like it's bouncing. The problem goes away when CanCollide is on or when the sphere is smaller than the character. Here's the code:

local forcefield = Instance.new("Part", Character)
forcefield.Name = "Forcefield"
forcefield.Shape = "Ball"
forcefield.Size = Vector3.new(8,8,8)
forcefield.CanCollide = false
forcefield.BrickColor = BrickColor.new("Teal")
forcefield.Transparency = 0.25
forcefield.TopSurface = Enum.SurfaceType.Smooth
forcefield.BottomSurface = Enum.SurfaceType.Smooth
local Weld = Instance.new("Weld", forcefield)
Weld.Part0 = Character.Torso
Weld.Part1 = forcefield
Weld.C1 = CFrame.new(0,0,0)

Is this a bug or am I doing something wrong?