ALTER TABLE

Hello!
Can someone explain me, why last ALTER fails?

work=# \d rdu
                                        Table "rdu"
   Column    |            Type             |                   Modifiers
-------------+-----------------------------+------------------------------------------------
 id          | integer                     | not null default
nextval('"rdu_id_seq"'::text)
 numurs      | integer                     |
 versija     | integer                     |
 klients_id  | integer                     |
 pasutitajs  | text                        |
 agents_id   | integer                     |
 pasut_nr    | integer                     |
 status_id   | character varying(1)        | default 'S'
 start_time  | timestamp without time zone | not null default now()
 end_time    | timestamp without time zone |
 finish_time | timestamp without time zone |
 apraksts    | text                        |
Indexes: rdu_agents_id,
         rdu_klients_id
Unique keys: rdu_id_key
Triggers: RI_ConstraintTrigger_1187649,
          RI_ConstraintTrigger_1187655,
          RI_ConstraintTrigger_1187673,
          RI_ConstraintTrigger_1187675

work=# ALTER TABLE rdu ADD CONSTRAINT rdu_end_time CHECK ((start_time <=
end_time) OR end_time IS NULL);
ALTER
work=# ALTER TABLE rdu ADD CONSTRAINT rdu_finish_time CHECK ((start_time <=
finish_time) OR finish_time IS NULL);
ERROR:  AlterTableAddConstraint: rejected due to CHECK constraint
rdu_finish_time

psql (PostgreSQL) 7.2.3

Mark