BUG: triggered data change violation on relation "tbl_b"

When I insert to table new row and after this in the same transaction I delete this row I get such error:
triggered data change violation on relation "tbl_b"

I've created database using simple script:

CREATE TABLE tbl_a
(
pn_id SERIAL,
pn_a VARCHAR(400) NOT NULL,
PRIMARY KEY (pn_id)
);
CREATE TABLE tbl_b
(
pc_id INT4 NOT NULL REFERENCES tbl_a (pn_id) ON UPDATE CASCADE ON DELETE CASCADE,
pc_b VARCHAR(40) NOT NULL,
PRIMARY KEY (pc_id, pc_b)
);
INSERT INTO tbl_a VALUES (1, 'xxx');

And this is the sample script that I use to generete this bug:

begin transaction;
insert into tbl_b values (1, 'xxx');
delete from tbl_b where pc_id=1;
ERROR: triggered data change violation on relation "tbl_b"

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