Error with DISTINCT

testdb=#SELECT DISTINCT table_2.f1, table_1.f2, '2000-08-22' AS
testdb-#date FROM table_1, table_2 WHERE table_1.f1 = table_2.f1;
ERROR: Unable to identify an ordering operator '<' for type 'unknown'
Use an explicit ordering operator or modify the query

I must execute this query :

testdb=#SELECT table_2.f1, table_1.f2, '2000-08-22' AS date FROM
testdb-#table_1, table_2 WHERE table_1.f1 = table_2.f1 GROUP BY
testdb-#table_2.f1, table_1.f2;

Is it a bug ?
Thanks in advance.

Jerome.