Partial index and query plan

Hello all,

Imagine having this table:

create table user_history (
rec_id SERIAL not null,
date TIMESTAMP not null,
action INT2 not null,
uid INT4 not null,
name CHAR(10) null default NULL,
constraint PK_USER_HISTORY primary key (rec_id),
constraint AK_DATE_USER_HIS unique (date)
);

and this partial index:

create unique index indx_date_action12_uid_user_his
on user_history (date, uid)
where action <> 0;

and this query:

select date
from "user_history"
where date > '2007-08-18 14:33'
and date <= '2007-08-18 16:30'
and uid = 1388
and action <> 0
limit 1;

The question is why "explain analyze" shows a 'Filter: ("action" <> 0)' in plan:

Limit (cost=0.00..3.05 rows=1 width=8) (actual time=4.798..4.798
rows=0 loops=1)
-> Index Scan using indx_date_action12_uid_user_his on
user_history (cost=0.00..6.10 rows=2 width=8) (actual
time=4.791..4.791 rows=0 loops=1)
Index Cond: ((date > '2007-08-18 14:33:40.60664'::timestamp
without time zone) AND (date <= '2007-08-18 16:30:00'::timestamp
without time zone) AND (uid = 138658))
Filter: ("action" <> 0)

when this is a "where" case of the index?