This used to work: \definefontfallback[seguiFB] [file:seguisym.ttf] [0x00000-0xFFFFF] [check=yes,force=no] \definetypeface [contextstart] [ss] [sans] [segoe] [default][fallbacks=seguiFB] (Intention is to take all missing characters from this symbol font.) But with current LMTX, I get: tex error > tex error on line 48 in file env_contextstart.tex: lua error: registered function call [1145]: ...-14/tex/texmf-context/tex/context/base/mkxl/math-act.lmt:2339: attempt to perform arithmetic on a nil value (field 'depth') stack traceback: ...-14/tex/texmf-context/tex/context/base/mkxl/math-act.lmt:2339: in local 'action' ...-14/tex/texmf-context/tex/context/base/mkxl/math-act.lmt:3166: in upvalue 'applytweaks' ...-14/tex/texmf-context/tex/context/base/mkxl/math-act.lmt:3200: in upvalue 'mathematics_tweakaftercopyingfont' [string "local mathematics_tweakaftercopyingfont = mat..."]:5: in local 'runner' ...-14/tex/texmf-context/tex/context/base/mkxl/font-ctx.lmt:504: in field 'aftercopyingcharacters' ...-14/tex/texmf-context/tex/context/base/mkxl/font-con.lmt:859: in field 'scale' ...-14/tex/texmf-context/tex/context/base/mkxl/font-otl.lmt:672: in function <...-14/tex/texmf-context/tex/context/base/mkxl/font-otl.lmt:664> (...tail calls...) ...-14/tex/texmf-context/tex/context/base/mkxl/font-def.lmt:378: in function <...-14/tex/texmf-context/tex/context/base/mkxl/font-def.lmt:366> (...tail calls...) ...-14/tex/texmf-context/tex/context/base/mkxl/font-def.lmt:484: in field 'read' ...-14/tex/texmf-context/tex/context/base/mkxl/font-ctx.lmt:1371: in function <...-14/tex/texmf-context/tex/context/base/mkxl/font-ctx.lmt:1228> (...tail calls...) 43 \usetypescriptfile[contextstart] 44 \usebodyfont[modern] 45 \usebodyfont[cambria] 46 \usebodyfont[segoe] 47 48 >> \setupbodyfont[contextstart,10pt] Hraban