SERIAL type's sequence is double-called or ?

Hello all,

I have a question with the SERIAL type. I want to use it for default
identification of table entries:

create table chuwee (
num serial primary key,
mesg varchar(50) not null
);

And all the inserts to this table I'm gonna log to this table:

create table chuwee_log (
id serial primary key,
date timestamp default current_timestamp,
num integer not null,
mesg varchar(50) not null
);

Logging will be done with this rule:

create or replace rule chuwee_rule as on insert to chuwee
do insert into chuwee_log (num, mesg)
values (new.num, new.mesg);

And all seems to work, but a sequence is called twice for inserts, and
logged "num"-s get a +1 value:

sunline=# insert into chuwee (mesg) values ('Hello, world!');
INSERT 9596671 1
sunline=# insert into chuwee (mesg) values ('Aquarium full of tequilla');
INSERT 9596673 1
sunline=# select * from chuwee_log;
id | date | num | mesg
----+----------------------------+-----+---------------------------
1 | 2007-09-08 16:18:26.707384 | 2 | Hello, world!
2 | 2007-09-08 16:18:58.711599 | 4 | Aquarium full of tequilla
(2 rows)

sunline=# select * from chuwee;
num | mesg
-----+---------------------------
1 | Hello, world!
3 | Aquarium full of tequilla
(2 rows)

How it can be cured, if any?

Best dishes,
Aleksandr.