getting sequence value in trigger

[puneet]

Hi
I have created the following trigger , It is created without any problems. But when I insert a row in tha table , I get "Cnnot insert null vale in column "
Plase let me know whats wrong or whats missing

CREATE OR REPLACE TRIGGER RF_PROD_BOM_INS_TRG
BEFORE INSERT ON RF_PRODUCT_BOM
FOR EACH ROW
BEGIN
UPDATE RF_PRODUCT_BOM SET PBRFNBR = (SELECT F_PROD_BOM_SEQ.NEXTVAL FROM DUAL );
END;
/

regards