Unique index problem

Hello

I created a unique index that doesn't seem to work when one column is
NULL.
Index is created like this: CREATE UNIQUE INDEX
lab_tests_groups_siui_uni ON lab_tests_groups_siui(id_lab_tests_siui,
id_lab_tests_groups, valid_from, id_lab_sample_types);
Now I can run this insert twice and I will have 2 records in the
database that seem to violate this index:
INSERT INTO lab_tests_groups_siui(id_lab_tests_siui,
id_lab_tests_groups, valid_from) VALUES(463, 9183, '2014-06-01');

When I create the index like this "CREATE UNIQUE INDEX
lab_tests_groups_siui_uni ON lab_tests_groups_siui(id_lab_tests_siui,
id_lab_tests_groups, valid_from);" index works fine.

I tested this on postgres 9.1.4 and 9.1.9.

Thank you.