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.