Asynchronous connection to socket always appears to succeed?

Pike 8.1 on Linux.

The following script ought to come through to connfailed() in the
event that there is no TELNET server on localhost. Instead, it comes
through to connected(), then afterward to sockclosed(). Am I doing
something stupidly wrong with asynchronous connections here?

object sock;

void sockread(mixed dummy,string data) {write("Data received: %O\n",data);}
void sockwrite() {write("Socket available for writing\n");}
void sockclosed() {write("Socket closed, terminating.\n"); exit(0);}

void connected()
{
    write("Connection appears successful [errno is %d]\n",sock->errno());
    sock->set_nonblocking(sockread,sockwrite,sockclosed);
}

void connfailed()
{
    write("Error connecting: %s [%d]\n",strerror(sock->errno()),sock->errno());
    sock->close();
    sockclosed();
}

int main()
{
    sock=Stdio.File(); sock->open_socket();
    sock->set_nonblocking(0,connected,connfailed);
    sock->connect("127.0.0.1",23);
    return -1;
}

ChrisA