Hello, LuaTeX doesn't segfault on invalid UTF-8 appearing in a file: this simply leads to a fatal error. However, one can build invalid UTF-8 using the bytes ^^^^^^110000 to ^^^^^^1100ff. If one builds a control sequence whose name is invalid UTF-8, then try to convert this sequence to a string, LuaTeX segfaults. Namely, both \expandafter\string\csname ^^^^^^1100ff\endcsname and \number\expandafter`\csname ^^^^^^1100ff\endcsname make LuaTeX segfault, but \edef\foo{\csname ^^^^^^1100ff\endcsname} \show\foo does not (even though the invalid UTF-8 name is written to the terminal). Regards, Bruno