plsql - PL/SQL: trigger error on altering insert data -

i'm trying write trigger table of books. table,

create table books (     isbn varchar(13) check (length (isbn) = 10 or length (isbn) = 13),     ...     primary key (isbn) ); 

i want write trigger when isbn of length 10 inserted, change format 13-digit 1 according following rules:

1. last digit removed
2. added 978 front

3. added check bit calculated formula

   check_bit = (10 - (x1 + 3*x2 + x3 + 3*x4 + ... + x11 + 3*x12) mod 10) mod 10

code:

create or replace trigger isbnformatchange before insert on books each row begin   if (length (:new.isbn) = 10)     :new.isbn := substr (:new.isbn, 1, 9);     :new.isbn := concat (978, :new.isbn);     :new.isbn := concat (:new.isbn, mod ((10 - mod ((to_number (substr (:new.isbn, 1, 1)) + 3 * to_number (substr (:new.isbn, 2, 1)) + to_number (substr (:new.isbn, 3, 1)) + 3 * to_number (substr (:new.isbn, 4, 1)) + to_number (substr (:new.isbn, 5, 1)) + 3 * to_number (substr (:new.isbn, 6, 1)) + to_number (substr (:new.isbn, 7, 1)) + 3 * to_number (substr (:new.isbn, 8, 1)) + to_number (substr (:new.isbn, 9, 1)) + 3 * to_number (substr (:new.isbn, 10, 1)) + to_number (substr (:new.isbn, 11, 1)) + 3 * to_number (substr (:new.isbn, 12, 1))), 10)), 10));   end if; end; 

but gives following error:

error(5,5): pl/sql: statement ignored error(5,63): pls-00330: invalid use of type name or subtype name 

i think did step 3 wrong (formula part)

function convert used converting different character types each other. convert character string number use function to_number. need replace in code convert (int, ...) to_number() function:

create or replace trigger isbnformatchange before insert on books each row begin   if (length (:new.isbn) = 10)     :new.isbn := substr (:new.isbn, 1, 9);     :new.isbn := concat (978, :new.isbn);     :new.isbn := concat (:new.isbn, mod ((10 - mod ((to_number(substr (:new.isbn, 1, 1)) + 3 * to_number(substr (:new.isbn, 2, 1)) + to_number(substr (:new.isbn, 3, 1)) + 3 * to_number(substr (:new.isbn, 4, 1)) + to_number( substr (:new.isbn, 5, 1)) + 3 * to_number( substr (:new.isbn, 6, 1)) + to_number(substr (:new.isbn, 7, 1)) + 3 * to_number( substr (:new.isbn, 8, 1)) + to_number( substr (:new.isbn, 9, 1)) + 3 * to_number( substr (:new.isbn, 10, 1)) + to_number(substr (:new.isbn, 11, 1)) + 3 * to_number( substr (:new.isbn, 12, 1))), 10)), 10));   end if; end; / 

convert function in documentation: http://docs.oracle.com/cd/b28359_01/server.111/b28286/functions027.htm#sqlrf00620
to_number function in documentation: http://docs.oracle.com/cd/e11882_01/server.112/e41084/functions211.htm#sqlrf06140