Casting to subtype in Oracle SQL when using refs

I have a type `lecturer_typ` with 2 sub types `module_leader_typ` and `external_typ`. I have inserted objects of all 3 of these types in one table called `lecturer_tbl`. I want to insert a reference to a `module_leader_typ` in another table but it's telling me: ``` SQL Error: ORA-00932: inconsistent datatypes: expected REF 21010175.MODULE_LEADER_TYP got REF 21010175.LECTURER_TYP' ``` Here's my code: ``` CREATE TYPE lecturer_typ AS OBJECT ( teacher_id NUMBER, first_name VARCHAR2(70), second_name VARCHAR2(70) ) NOT FINAL; CREATE TYPE module_leader_typ UNDER lecturer_typ ( subject_knowledge VARCHAR2(255), contract_type VARCHAR(100), years_experience NUMBER(3) ); CREATE TYPE external_lecturer_typ UNDER lecturer_typ ( institution VARCHAR2(255), is_examiner CHAR(1) -- CONSTRAIN to 1 or 0 ); create table lecturer_tbl of lecturer_typ; INSERT INTO lecturer_tbl VALUES( lecturer_typ(1,'jack','parrow') ); INSERT INTO lecturer_tbl VALUES( lecturer_typ(2,'liz','taylor') ); INSERT INTO lecturer_tbl VALUES ( external_lecturer_typ(51,'noddy','friend','cambridge','1') ); INSERT INTO lecturer_tbl VALUES ( external_lecturer_typ(52,'big','ears','oxford','1') ); INSERT INTO lecturer_tbl VALUES ( external_lecturer_typ(53,'mike','strutter','hull','1') ); INSERT INTO lecturer_tbl VALUES ( external_lecturer_typ(54,'john','ross','brunel','1') ); INSERT INTO lecturer_tbl VALUES ( external_lecturer_typ(55,'special','friend','berks','1') ); INSERT INTO lecturer_tbl VALUES ( module_leader_typ(31,'bollin','ceake','networking','full-time',30) ); INSERT INTO lecturer_tbl VALUES ( module_leader_typ(32,'biz','silovoxi','databases','full-time',15) ); INSERT INTO lecturer_tbl VALUES ( module_leader_typ(33,'tony','blair','Web Technologies','part-time',3) ); Create table module_leaders_tbl ( the_leader REF module_leader_typ ); insert into module_leaders_tbl VALUES ( (SELECT REF(l) FROM lecturer_tbl l where l.teacher_id = 31) ); ```