Use of GROUP BY - Mailing list pgsql-general

PG7.2.1 on RH7.3
There is no 1234 value in field1.
select sum(amount) from table1 where field1 = 1234;
 sum
-----

(1 row)

select sum(amount) from table1 where field1 = 1234 group by field1;
 sum
-----
(0 rows)

If GROUP BY should always be used in statements using aggregate functions,
why don't I get an error?

Thank you all.