Trouble with foreign key

Hi,

there is a problem with foreign key (PG 8.3RC2). Please, look at below. I don't understand this behavior. Why this
INSERTis ending with this error message. In my opinion, the key is obviously present. Thank you. 
--
Otakarek

# INSERT INTO www_es_orderhead_cs(_status,_tmp_ip,_tmp_stamp)
VALUES('n','xx.121.111.31','2f980de9f2297c7902f3415f6537c6be');

ERROR:  insert or update on table "www_es_orderhead_cs" violates foreign key constraint
"www_es_orderhead_cs__tmp_stamp_fkey"
DETAIL:  Key (_tmp_stamp)=(2f980de9f2297c7902f3415f6537c6be) is not present in table "tmp_stamp".

# SELECT * from tmp_stamp;
       _ip        |              _stamp              |           _expired            | _var
------------------+----------------------------------+-------------------------------+------
 xx.121.111.31/32 | 2f980de9f2297c7902f3415f6537c6be | 2008-04-08 17:49:33.193914+02 |
(1 row)