why it doesn't work? referential integrity

Hello

I found strange postgresql's behave. Can somebody explain it?

Regards
Pavel Stehule

CREATE TABLE users (
id integer NOT NULL,
name VARCHAR NOT NULL,
PRIMARY KEY (id)
);

INSERT INTO users VALUES (1, 'Jozko');
INSERT INTO users VALUES (2, 'Ferko');
INSERT INTO users VALUES (3, 'Samko');

CREATE TABLE tasks (
id integer NOT NULL,
owner INT REFERENCES users (id) ON UPDATE CASCADE ON DELETE SET NULL,
worker INT REFERENCES users (id) ON UPDATE CASCADE ON DELETE SET NULL,
checked_by INT REFERENCES users (id) ON UPDATE CASCADE ON DELETE SET NULL,
PRIMARY KEY (id)
);
INSERT INTO tasks VALUES (1,1,NULL,NULL);
INSERT INTO tasks VALUES (2,2,2,NULL);
INSERT INTO tasks VALUES (3,3,3,3);

DELETE FROM users WHERE id = 1; -- works simple
DELETE FROM users WHERE id = 2; -- works ok
DELETE FROM users WHERE id = 3; -- doesn't work, why?

ERROR: insert or update on table "tasks" violates foreign key
constraint "tasks_checked_by_fkey"
DETAIL: Key (checked_by)=(3) is not present in table "users".
CONTEXT: SQL statement "UPDATE ONLY "public"."tasks" SET "worker" =
NULL WHERE $1 OPERATOR(pg_catalog.=) "worker""