Hi,
I had the same error in my addon gmFonts and I have modified all the call adding a "" ( a void string ) to fill where there is a nil but it should not.
like:
Lua Code:
SetFont(_G.QuestFontNormalSmall, GMFONTS["B"], 13, "", nil, nil, nil, 0.54, 0.4, 0.1)
Let me know.