Insert ref data into nested table

[singjiu]

I have listed some of the important codes.

create type subject_type as object
(subject_id char(7), subject_name varchar(50), semester integer, location
varchar(20), day_taught date, sub_school ref school_type);

create type sub_ref_type as object
(subj ref subject_type, semester date);

create type subject_list_type as table of sub_ref_type;

create type lecturer_type under person_type
(lecturer_id char(6), start_date timestamp(3), qualification varchar(50),
school_belong ref school_type, teaching_subject subject_list_type, member
function getTeachingY return interval year to month);

create table lecturer_objtab of lecturer_type
(lecturer_id constraint lecturer_id_pk_t primary key, school_belong scope is
school_objtab) nested table teaching_subject store as teaching_subject_list;

insert into lecturer_objtab (lecturer_id, name, address, dob, gender,
start_date, qualification, contact_num, school_belong, teaching_subject) values
('L02356', 'Chong Ee Vern', address_type('93, Kampung Salang',
'Kubang Pasu', '06150', 'Kedah'), to_date('05-12-1975',
'dd-mm-yyyy'), 'F', to_date('05-1998','mm-yyyy'),
'Master of Business Adminstration', contact_list_type('04-7940212'),
(select ref(s) from school_objtab s where school_id = 'BUS'),
subject_list_type()
);

insert into table (select l.teaching_subject from lecturer_objtab l where l.lecturer_id =
'L02356')
select ref(sub), to_date('1-2001', 'mm-yyyy') from subject_objtab sub
where sub.subject_id = 'CF1011' ;


When I insert the final statement, an error message occurs:-
"cannot INSERT object view REF or user-defined REF"

Please help me, thank you