[Bug] How to crash a Mac!

One of my testers reported that he experiences crashes in Mac OS 10.4 when running my addon. I've tracked it down to this piece of code, which works fine on Windows.

local tooltip = CreateFrame("GameTooltip")
tooltip:SetOwner(UIParent, "ANCHOR_NONE")
local right1, right2 = tooltip:CreateFontString(), tooltip:CreateFontString()
local duplicate = tooltip:CreateFontString()
tooltip:AddFontStrings(duplicate, duplicate)
tooltip:AddFontStrings(duplicate, right1)
tooltip:AddFontStrings(duplicate, right2)

I removed the 'duplicate' font string and just created a new instance instead of assigning the same one and the crashes are gone.