Why does my UDim2 not size smaller than .5?

local corelength = 1
script.Parent.Humanoid.HealthChanged:Connect(function()
	local corelength = script.Parent.Humanoid.Health/script.Parent.Humanoid.MaxHealth
	script.Parent.Head.BillboardGui.Health.Inside.Size =  UDim2.new(corelength,0,0,20)
end)

When resizing a image in a BillboardGui it disappears if sized under .5, anyone got an idea of why?