Hi, when you pass a undefined argument to \lfence (or \rfence and \mfence) you get the "unknown" alternative \setvalue{\??mathleft \s!unknown}{\setfalse\c_math_fenced_done\ifconditional\c_math_fenced_unknown\normalleft \nexttoken\fi} which doesn’t work here because \nexttoken is never set by \lfence but only by \left. Wolfgang