SQL & PL/SQL

Hi all.

ORA-20001 is allegedly a user-defined error, but I haven't defined any errors for my ad hoc program. Oracle SQL Developer appears to be having a problem with my use of SYS.ORA_OPT_LOCK.CHECKSUM.

Can anyone tell me about, or spot any obvious errors / omissions? See Program and called Function below

I'm NOT getting ORA-06502: PL/SQL: numeric or value error: character string buffer too small

ORA-06512 is complaining about SYS.OWA_OPT_LOCK at lines 38 & 120.

Regards,
Colin

Program:
DECLARE
j NUMBER;
x VARCHAR2(20) := 'PRODDTA';
y VARCHAR2(20) := 'BILL2CUS_CHCK';
BEGIN
FOR i IN (SELECT ROWID
FROM BILL2CUS_CHCK)
LOOP
j := c_sum(x,y,i.ROWID);
update_BILL2CUSCHCK(j,i.ROWID);
END LOOP;
END;

Function:
create or replace FUNCTION C_SUM(p_owner IN VARCHAR2,p_tname IN VARCHAR2,p_rowid IN ROWID)
RETURN NUMBER
IS
BEGIN
RETURN OWA_OPT_LOCK.CHECKSUM(p_owner,p_tname,p_rowid);
END;