Part moving via script teleports when stuff is in the way?

It's supposed to be a wall that comes up from the ground, but when it touches the bottom of the ground, it teleports to the top.

  IWall.Ice.CFrame = Player.Character.Torso.CFrame*CFrame.new(0, -20, -15)
           for i = 1,20 do
	       IWall.Ice.Position = IWall.Ice.Position + Vector3.new(0,1,0)
           wait (0.1)
end