serial type as foreign key referential integrity violation

Hi all,     Just briefly describe my problem.     I have two tables.
create table A(  a1 serial primary key,  a2 varchars(10)
);
create table B(   b1 integer primary key,   b2 Integer,   foreign key(b2) references a(a1)
)
insert into A values('123'); 
select a1 from A where a2='123'
>--
>a1 
>--
>1
>--
insert into B values (1,1);
ERROR!! referential integrity violation - key referenced from B not found in A.

but in table A , if I change it the PK to integer, everything would be fine.

any idea?

thanks a lot!