Bug regarding update.. from and aggregates ?

Hi

When executing the following statements:
create schema bugtest ;
drop table bugtest.t1 ;
drop table bugtest.t2 ;
create table bugtest.t1(id int4, maximum int4, dummy int4) ;
create table bugtest.t2(id int4, entry int4) ;
insert into bugtest.t1 (id) values (1) ;
insert into bugtest.t1 (id) values (2) ;
insert into bugtest.t2 (id, entry) values (1, 1) ;
insert into bugtest.t2 (id, entry) values (1, 2) ;
insert into bugtest.t2 (id, entry) values (2, 3) ;
insert into bugtest.t2 (id, entry) values (2, 4) ;
update bugtest.t1 set maximum = max(t2.entry) from bugtest.t2
where t1.id = t2.id ;

The last update given "One row affected" - and indeed, t1 looks
like this after the update:
id | maximum
1 | 4
2 | <NULL>

While I would expect:
id | maximum
1 | 2
2 | 4

Is this I bug, or is the observed behaviour actually intended?

greetings, Florian Pflug