invalid key to 'next'

This is my lua code: http://lua.pastebin.com/f7b33bcca
It's in a function called createGUI().  I've omitted the function itself and
a few unneeded lines.  I call this function from C++ as such:

        try
        {
            luabind::call_function<void>(_lua, "createGUI",
boost::ref((gui::widget&)*_sheet));
        }
        catch(const luabind::error &e)
        {
            engine::log(engine::string(e.what()) + " :: " + _lua.error());
        }

This results in:

lua runtime error :: invalid key to 'next'

This is under Lua 5.1.4 and Luabind 0.8.  The same code worked 100%
correctly under Lua 5.0.x.  I cannot find what is wrong, any ideas?  Major
thanks to anyone with ideas!

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