A deceiving error message while setting a replication slave

I was setting up a slave manually for a quick-and-dirty experiment, when I found this one:

mysql  %gt; slave start;
ERROR 1200 (HY000): The server is not configured as slave; fix in config file or with CHANGE MASTER TO

Hmm... I did the CHANGE MASTER TO already, but just for the sake of it, let's do it again.

mysql %gt; change master to master_host='127.0.0.1', master_port=22346, 
master_user='msandbox', master_password='msandbox', 
master_log_file='mysql-bin.000002', master_log_pos=106;
Query OK, 0 rows affected (0.00 sec)

mysql  %gt; slave start;
ERROR 1200 (HY000): The server is not configured as slave; fix in config file or with CHANGE MASTER TO

Again! Why is it asking to execute CHANGE MASTER TO, which has been accepted right now?

Then it came to me. Because I did a manual configuration, and I didn't set the server_id.

mysql [localhost] {msandbox} ((none)) %gt; set global server_id=4000;
Query OK, 0 rows affected (0.00 sec)
mysql  %gt; slave start;
Query OK, 0 rows affected (0.00 sec)

Yep. That was it. The error message could have been more helpful, though.