SQL & PL/SQL

There is a need to read match and update data from and into a custom table. The table would have about 3 millions rows and holds key numbers. BAsed on a field value of this custom table, relevant data needs to be fetched from joins of other tables and updated in the custom table. I plan to use BULK COLLECT and FORALL.
All examples I have seen, do an insert into a table. How do I go about reading all values of a given field and fetching other relevant data and then updating the custom table with data fetched.

Defined an object with specifics like this

CREATE OR REPLACE TYPE imei_ot AS OBJECT (
recid NUMBER,
imei VARCHAR2(30),
STORE VARCHAR2(100),
status VARCHAR2(1),
TIMESTAMP DATE,
order_number VARCHAR2(30),
order_type VARCHAR2(30),
sku VARCHAR2(30),
order_date DATE,
attribute1 VARCHAR2(240),
market VARCHAR2(240),
processed_flag VARCHAR2(1),
last_update_date DATE
);

Now within a package procedure I have defined like this.

type imei_ott is table of imei_ot;
imei_ntt imei_ott;
begin
SELECT imei_ot (recid,
imei,
STORE,
status,
TIMESTAMP,
order_number,
order_type,
sku,
order_date,
attribute1,
market,
processed_flag,
last_update_date
)
BULK COLLECT INTO imei_ntt
FROM (SELECT stg.recid, stg.imei, cip.store_location, 'S',
co.rtl_txn_timestamp, co.rtl_order_number, 'CUST',
msi.segment1 || '.' || msi.segment3,
TRUNC (co.txn_timestamp), col.part_number, 'ZZ',
stg.processed_flag, SYSDATE
FROM custom_orders co,
custom_order_lines col,
custom_stg stg,
mtl_system_items_b msi
WHERE co.header_id = col.header_id
AND msi.inventory_item_id = col.inventory_item_id
AND msi.organization_id =
(SELECT organization_id
FROM hr_all_organization_units_tl
WHERE NAME = 'Item Master'
AND source_lang = USERENV ('LANG'))
AND stg.imei = col.serial_number
AND stg.processed_flag = 'U');

/* Update staging table in one go for COR order data */
FORALL indx IN 1 .. imei_ntt.COUNT
UPDATE custom_stg
SET STORE = TREAT (imei_ntt (indx) AS imei_ot).STORE,
status = TREAT (imei_ntt (indx) AS imei_ot).status,
TIMESTAMP = TREAT (imei_ntt (indx) AS imei_ot).TIMESTAMP,
order_number = TREAT (imei_ntt (indx) AS imei_ot).order_number,
order_type = TREAT (imei_ntt (indx) AS imei_ot).order_type,
sku = TREAT (imei_ntt (indx) AS imei_ot).sku,
order_date = TREAT (imei_ntt (indx) AS imei_ot).order_date,
attribute1 = TREAT (imei_ntt (indx) AS imei_ot).attribute1,
market = TREAT (imei_ntt (indx) AS imei_ot).market,
processed_flag =
TREAT (imei_ntt (indx) AS imei_ot).processed_flag,
last_update_date =
TREAT (imei_ntt (indx) AS imei_ot).last_update_date
WHERE recid = TREAT (imei_ntt (indx) AS imei_ot).recid
AND imei = TREAT (imei_ntt (indx) AS imei_ot).imei;

DBMS_OUTPUT.put_line ( TO_CHAR (SQL%ROWCOUNT)
|| ' rows updated using Bulk Collect / For All.'
);
EXCEPTION
WHEN NO_DATA_FOUND
THEN
DBMS_OUTPUT.put_line ('No Data: ' || SQLERRM);
WHEN OTHERS
THEN
DBMS_OUTPUT.put_line ('Other Error: ' || SQLERRM);
END;



Now for the unfortunate part. When I compile the pkg, I face an error
PL/SQL: ORA-00904: "LAST_UPDATE_DATE": invalid identifier

I am not sure where I am wrong. Object type has the last update date field and the custom table also has the same field.
Could someone please throw some light and suggestion?

Thanks
uds