SQL & PL/SQL

I got the Learn Oracle from Oracle book and been following few of the steps in order to learn Oracle.
I wrote the next sintax in PL/SQL, and got an error message. Do you know how to declare the 'CUSTOMERS.CUSTOMER_ID' ?

SQL> CREATE PROCEDURE new_cust
2 (p_id IN customers.customer_id%TYPE
3 , p_lastname IN customers.cust_last_name%TYPE
4 , p_fisrtname IN customers.cust_first_name%TYPE)
5 IS
6 BEGIN
7 INSERT INTO customers
8 (customer_id, cust_last_name, cust_first_name)
9 VALUES
10 (p_id, p_lastname, p_firstname);
11 COMMIT;
12 END new_cust;
13 /

Advertencia: Procedimiento creado con errores de compilación.

SQL> SHOW ERROR
Errors para PROCEDURE NEW_CUST:

LINE/COL ERROR
-------- -----------------------------------------------------------------
0/0 PL/SQL: Compilation unit analysis terminated
2/10 PLS-00201: identifier 'CUSTOMERS.CUSTOMER_ID' must be declared

Thanks!