Transaction isolation with concurrent increments

Well, I'm a relative newbie to SQL, but I have been told that this behaviour
is a bug. I hope I didn't misunderstand something.

To reproduce, first create a table 'counters' that looks like that:

counter1
----------
0
(1 row)

Then execute the following commands parallelly on two psql terminals:

set transaction isolation level serializable;
begin;
update counters set counter1=(select counter1+1 from counters);

(The second psql terminal will not return from executing the last statement)

Commit on both terminals, then do a select again. The result will be:

counter1
----------
1
(1 row)

Oops! ;-)

If instead I use an update statement like that:

update counters set counter1=counter1+1;

psql behaves like expected, i.E. the result is 2.

--
Grüße,

Nico