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db_debug has these lines:

    if (luaL_loadbuffer(L, buffer, strlen(buffer), "=(debug command)") ||
        lua_pcall(L, 0, 0, 0))
      lua_writestringerror("%s\n", lua_tostring(L, -1));

lua_writestringerror is defined as:

#define lua_writestringerror(s,p) \
        (fprintf(stderr, (s), (p)), fflush(stderr))

lua_tostring may return NULL if the error object that lua_pcall pushes onto the
stack is not a string. In this case NULL is passed to fprintf, which is
undefined behavior.


Lua 5.4.0  Copyright (C) 1994-2020, PUC-Rio
> debug.debug()
lua_debug> error({})