are there gotchas controlling varargs anxiety parameters

i have square formula (summarized)...

ansistring working(ansistring format,...)
{
    va_list argptr;
    ansistring buff;

    va_start(argptr, format);
    buff.vprintf(format.c_str(), argptr);

    va_end(argptr);
    relapse buff;
}

and, basement pass anxiety comparison where possible, i altered thusly.

ansistring broken(const ansistring &format,...)
{
... rest, totally relating ...
}

my job formula this:-

ansistring s1, s2;
    s1 = working("hello %s", "world");
    s2 = broken("hello %s", "world");

but, s1 contains "hello world", while s2 "hello (null)". i cruise due proceed va_start works, nonetheless i'm accurately certain what's going on.