LuaLaTex using fontspec package and luacode reading JSON file - json

Question

I'm using Latex since years but I'm new to embedded luacode (with Lualatex). Below you can see a simplified example:
\begin{filecontents*}{data.json}
[
{"firstName":"Max", "lastName":"Möller"},
{"firstName":"Anna", "lastName":"Smith"}
];
\end{filecontents*}
\documentclass[11pt]{article}
\usepackage{fontspec}
%\setmainfont{Carlito}
\usepackage{tikz}
\usepackage{luacode}
\begin{document}
\begin{luacode}
require("lualibs.lua")
local file = io.open('data.json','rb')
local jsonstring = file:read('*a')
file.close()
local jsondata = utilities.json.tolua(jsonstring)
tex.print('\\begin{tabular}{cc}')
for key, value in pairs(jsondata) do
tex.print(value["firstName"] .. ' & ' .. value["lastName"] .. '\\\\')
end
tex.print('\\hline\\end{tabular}')
\end{luacode}
\end{document}
When executing Lualatex following error occurs:
LuaTeX error [\directlua]:6: attempt to index field 'json' (a nil value) [\directlua]:6: in main chunk. \end{luacode}
When commenting the line \usepackage{fontspec} the output will be produced. Alternatively, the error can be avoided by commenting utilities.json.tolua(jsonstring) and all following lua-code lines.
So the question is: How can I use both "fontspec" package and json-data without generating an error message? Apart from this I have another question: How to enable german umlauts in output of luacode (see first "lastName" in example: Möller)?
Ah, I'm using TeX Live 2015/Debian on Ubuntu 16.04.
Thank you,
Jerome