Есть триггер:
CREATE TRIGGER NEW_TK FOR NORMA ACTIVE
AFTER INSERT POSITION 0
AS
declare variable CNT INTEGER;
BEGIN
SELECT COUNT (*)
FROM skl_tk
WHERE skl_tk.cod=NEW.cod
INTO :CNT;
if (:CNT=0) then
INSERT INTO SKL_TK (COD, ART, DIZ, COL)
VALUES (NEW.COD, NEW.ART, NEW.DIZ, NEW.COL);
END

Во время выполнения выдаётся ошибка "validation error for column ART, value "***null***"". В чём проблема?
Структуры таблиц:

CREATE TABLE SKL_TK (
ART VARCHAR (30) character set WIN1251 NOT NULL collate WIN1251,
DIZ VARCHAR (30) character set WIN1251 collate WIN1251,
COL VARCHAR (30) character set WIN1251 NOT NULL collate WIN1251,
NAL NUMERIC (15, 2) NOT NULL,
SEBEST NUMERIC (15, 2),
COD VARCHAR (12) character set WIN1251 NOT NULL collate WIN1251);

CREATE TABLE NORMA (
COD VARCHAR (12) character set WIN1251 NOT NULL collate WIN1251,
NAMEPR VARCHAR (30) character set WIN1251 NOT NULL collate WIN1251,
COLPR VARCHAR (30) character set WIN1251 NOT NULL collate WIN1251,
SEBEST NUMERIC (15, 2),
NORM NUMERIC (15, 4) NOT NULL,
ART VARCHAR (30) character set WIN1251 NOT NULL collate WIN1251,
DIZ VARCHAR (30) character set WIN1251 collate WIN1251,
COL VARCHAR (30) character set WIN1251 collate WIN1251);