pdflatex error for TikZ externlization when using LuaLaTeX

Question

This is how I am calling LuaLaTeX in TeXstudio:

lualatex.exe -synctex=1 -interaction=nonstopmode --shell-escape %.tex

Doing a test run of my main document, I get this error throughout the document (in all the log files for the externalized TikZ images):

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
! Fatal fontspec error: "cannot-use-pdftex"
! 
! The fontspec package requires either XeTeX or LuaTeX to function.
! 
! You must change your typesetting engine to, e.g., "xelatex" or "lualatex"
! instead of plain "latex" or "pdflatex".
! 
! See the fontspec documentation for further information.
! 
! For immediate help type H <return>.
!...............................................  

This is my configuration file for fonts:

\usepackage{fontspec}
\usepackage{microtype}
\usepackage{amsmath}

%%%%%%%%
\usepackage{unicode-math}
\setmainfont{TeX Gyre Termes}
\setmathfont{TeX Gyre Termes Math}
\setsansfont[Scale=MatchUppercase]{TeX Gyre Heros}

There is no further line in my document where inputenc or any other problematic tidbit is existent.

Basically, I am clueless. How can I proceed here?

SOLUTION (edit)

One has to add:

\tikzset{
external/system call={lualatex \tikzexternalcheckshellescape -halt-on-error -interaction=batchmode -jobname "\image" "\texsource"},
}