I have no idea how to transpose the following latex example to context. Also in my opinion the input is wrong, it should be written as \mathtestbase{fb}^{-1} (and this works fine). So I'm only posting this for the case that it indicates some hidden problem in the fontloader. The error I get from the document below is ! error: (nodes): attempt to double-free math_char node 100300, ignored ! ==> Fatal error occurred, no output PDF file produced! The fonts used are [latinmodern-math.otf]:mode=base;script=math;language=DFLT; and STIXTwoText:mode=base;script=latn;language=DFLT; Imho the relevant points to trigger the error are - a math alphabet which uses mode=base - a ligature in this font (fb here) - a following math symbol directly behind the ligature (\int would give an error too). \documentclass{article} \usepackage{unicode-math} \setmathfontface\mathtestbase{STIX Two Text}[Renderer=Basic] \begin{document} $\mathtestbase{fb^{-1}}$ \end{document} -- Ulrike Fischer http://www.troubleshooting-tex.de/