SQL & PL/SQL

I have written the following function for finding the number of items in stock in the item table.

CREATE OR REPLACE function getAmount (ItemID IN NUMBER)
RETURN NUMBER
AS
invalid_id EXCEPTION;
returnedQty number;
BEGIN
Select qty
Into returnedQty
From item
Where itemNo = ItemID;
RETURN (returnedQty);
IF SQL%NOTFOUND THEN
RAISE invalid_id;
END IF;
COMMIT;
Exception
WHEN invalid_id THEN
DBMS_OUTPUT.PUT_LINE('Invalid ID entered');
END getAmount;

The function compiles successfully, although there is a problem that Oracle is not handling my user-defined exception invalid_id

If I use the following for a valid itemID:

DECLARE
return_value number;
BEGIN
return_value := getAmount(1);
DBMS_OUTPUT.PUT_LINE (return_value);
END;

then the function returns the quantity of items in stock correctly.

However, if I enter an incorrect itemID, say 20

DECLARE
return_value number;
BEGIN
return_value := getAmount(20);
DBMS_OUTPUT.PUT_LINE (return_value);
END;

The invalid_id exception is not raised, and the Oracle errors says: no_data_found and the function has not returned a value. If I add a no_data_found exception, this works perfectly, but for this assignment I must write my own user-defined error.

Any help would be very much appreciated!

Thank you.