Oracle Forms

My purpose is to commit the changes in the form and execute a custom procedure to update the price details in a custom form. The following are the details

BLOCK=> LINES
COLUMN=> RETURN_DF

1. On the WHEN-VALIDATE-ITEM event of the RETURN_DF column, i am setting a variable (DUMMY) to "Y".

2. Created a form level trigger to watch the status of the DUMMY column, if the dummy is set to "Y" then commit the changes and execute the custom procedure to update the price details.

The following is the code in the form level trigger

DECLARE
l_header_id NUMBER:=NAME_IN('ORDER.HEADER_ID');
BEGIN
IF :LINE.DUMMY='Y' THEN
COMMIT;
:LINE.DUMMY:='N';
xx_custom.update_order_prices(l_header_id);
GO_BLOCK('LINES');
EXECUTE_QUERY;
END IF;
END;

Probelm
=======
In the above trigger, at the COMMIT statement, i am getting 100500: non ORACLE exception error.

What is wrong with the above statements..

Please post your replies.. Thanks in adavance...