update with subquery - unexpected results

I am using postgres 8.1.2 on a windows computer.
My postgres.config has:
add_missing_from = off

In the following example, the first update sets value for v1 in table
t1 to 4 for all rows.
Just for my sanity, I checked the null condition in the second update,
and this
does not update any of the rows, am I missing something?

create table t1 (v1 integer);
create table t2 (v2 integer);

insert into t1 (v1) values (1);
insert into t1 (v1) values (2);

insert into t2 (v2) values (3);

update t1 set v1=4 where v1 IN (select v1 from t2);

update t1 set v1=4 where v1 IN (select NULLIF(1,1) from t2);

Thanks,
Roger