question about unique indexes

-- running pg 8.4

i have a table defining geographic locations

    id
    lat
    long
    country_id not null
    state_id
    city_id
    postal_code_id

i was given a unique index on
    (country_id, state_id, city_id, postal_code_id)

the unique index isn't working as i'd expect it to.  i was hoping
someone could explain why:

in the two records below, only country_id and state_id are assigned
( aside from the serial )

geographic_location_id | coordinates_latitude | coordinates_longitude
| country_id | state_id | city_id | postal_code_id
------------------------+----------------------+-----------------------
+------------+----------+---------+----------------
                     312 |
|                       |        233 |       65 |         |
                     443 |
|                       |        233 |       65 |         |

i was under the expectation that the unique constraint would apply in
this place.

from the docs:
    When an index is declared unique, multiple table rows with equal
indexed values are not allowed. Null values are not considered equal.
A multicolumn unique index will only reject cases where all indexed
columns are equal in multiple rows.