\starttext \m{\root3\of2} \stoptext works. But \starttext \startMPcode label("\m{\root3\of2}", origin); \stopMPcode \stoptext doesn't. Running `LuaMetaTeX' on `trial' with ``/home/fvleung/ConTeXt/tex/texmf-linux-64/bin/mtxrun --autogenerate --script context trial.tex'' resolvers | formats | executing runner 'run luametatex format': /home/fvleung/ConTeXt/tex/texmf-linux-64/bin/luametatex --jobname="trial" --fmt=/home/fvleung/ConTeXt/tex/texmf-cache/luatex-cache/context/5fe67e0bfe781ce0dde776fb1556f32e/formats/luametatex/cont-en.fmt --lua=/home/fvleung/ConTeXt/tex/texmf-cache/luatex-cache/context/5fe67e0bfe781ce0dde776fb1556f32e/formats/luametatex/cont-en.lui cont-yes.mkiv --c:currentrun=1 --c:fulljobname="./trial.tex" --c:input="./trial.tex" --c:kindofrun=1 --c:maxnofruns=9 --c:texmfbinpath="/home/fvleung/ConTeXt/tex/texmf-linux-64/bin" open source > level 1, order 1, name 'cont-yes.mkiv' fonts > beware: no fonts are loaded yet, using 'lm mono' in box system > system > ConTeXt ver: 2021.07.27 19:27 LMTX fmt: 2021.7.28 int: english/english system > system > 'cont-new.mkxl' loaded open source > level 2, order 2, name '/home/fvleung/ConTeXt/tex/texmf-context/tex/context/base/mkxl/cont-new.mkxl' system > beware: some patches loaded from cont-new.mkiv close source > level 2, order 2, name '/home/fvleung/ConTeXt/tex/texmf-context/tex/context/base/mkxl/cont-new.mkxl' system > files > jobname './trial', input './trial.tex', result './trial' fonts > latin modern fonts are not preloaded languages > language 'en' is active open source > level 2, order 3, name './trial.tex' fonts > preloading latin modern fonts (second stage) fonts > 'fallback modern-designsize rm 12pt' is loaded metapost > initializing instance 'metafun:1' using format 'metafun' and method 'default' metapost > loading 'metafun' as 'metafun.mpxl' using method 'default' metapost > initializing number mode 'scaled' metapost > trace > This is MPLIB for LuaMetaTeX, version 3.11, running in scaled mode. metapost > trace > metapost > trace > loading metafun for lmtx, including the plain 1.004 base definitions metapost > trace > tex error > tex error on line 7 in file ./trial.tex: Argument of \root has an extra } 32\normalstopimath \fi <inserted text> \par <to be read again> } <lua output> } <local> \global \setbox \mptextbox \hbox {\clf_mptexttoks } <inserted text> [[special cmd: end local call]] <to be read again> \relax ... \stopMPcode 1 \starttext 2 3 \startMPcode 4 label("\m{\root3\of2}", origin); 5 \stopMPcode 6 7 >> \stoptext 8 I've run across a '}' that doesn't seem to match anything. For example, '\def\a#1{...}' and '\a}' would produce this error. The '\par' that I've just inserted will cause me to report a runaway argument that might be the root of the problem. mtx-context | fatal error: return code: 256 TeX Output exited abnormally with code 1 at Fri Sep 3 17:44:19