SQL & PL/SQL

Apologies as I'm aware this error has been raised before, however, after a couple of hours surfing and trying to find the reason I'm getting this error, I have been unable to resolve it for my particular case.

I've constructed a simplified example of my code, which basically involves declaring an object table, which contains a nested object table.  These represent headers and details, where each header will have one or more details.  As I'm new to objects, I'm simply trying to initialise these objects in the code and will later be using the initialised object to call a package procedure that will operate on the data.

Here's my simplified code:

create or replace TYPE DETAIL_OBJ_TYPE AS OBJECT
(
    fact_name          VARCHAR2(100),
    fact_type          VARCHAR2(20),
    fact_value         VARCHAR2(100)   
);

create or replace TYPE DETAILS_DATA_TBL AS TABLE OF DETAIL_OBJ_TYPE;

create or replace TYPE HEADER_OBJ_TYPE AS OBJECT
(
    data_set_name      VARCHAR2(50),
    effective_date     DATE,   
    distinct_data      VARCHAR2(1),
    indexed_value_list DETAILS_DATA_TBL
);

create or replace TYPE HEADERS_DATA_TBL AS TABLE OF HEADER_OBJ_TYPE;

declare    
  lo_headers_tbl HEADERS_DATA_TBL;

begin
  -- Initialise the object table
  lo_headers_tbl := HEADERS_DATA_TBL();

  -- Populate first header
  lo_headers_tbl.EXTEND;
  lo_headers_tbl(1).data_set_name := 'PRINT';             <-- Error reports this line
  lo_headers_tbl(1).effective_date := '20-MAY-16';
  lo_headers_tbl(1).distinct_data := 'Y';

  -- Populate details for first header    
  lo_headers_tbl(1).indexed_value_list.EXTEND;
  lo_headers_tbl(1).indexed_value_list(1) := DETAIL_OBJ_TYPE ('INDEXED_FACT_1','VARCHAR2','SB');
              
  lo_headers_tbl(1).indexed_value_list.EXTEND;
  lo_headers_tbl(1).indexed_value_list(2) := DETAIL_OBJ_TYPE ('INDEXED_FACT_2','VARCHAR2','DS');
              
  lo_headers_tbl(1).indexed_value_list.EXTEND;
  lo_headers_tbl(1).indexed_value_list(3) := DETAIL_OBJ_TYPE ('INDEXED_FACT_4','NUMBER','1');
              
  lo_headers_tbl(1).indexed_value_list.EXTEND;
  lo_headers_tbl(1).indexed_value_list(4) := DETAIL_OBJ_TYPE ('INDEXED_FACT_7','DATE','23-MAY-2016');


  -- Populate second header
  lo_headers_tbl.EXTEND;
  lo_headers_tbl(2).data_set_name := 'SCREEN';
  lo_headers_tbl(2).effective_date := '28-MAY-16';
  lo_headers_tbl(2).distinct_data := 'Y';
    
  -- Populate details for second header    
  lo_headers_tbl(2).indexed_value_list.EXTEND;
  lo_headers_tbl(2).indexed_value_list(1) := DETAIL_OBJ_TYPE ('INDEXED_FACT_1','VARCHAR2','AER02');
              
  lo_headers_tbl(2).indexed_value_list.EXTEND;
  lo_headers_tbl(2).indexed_value_list(2) := DETAIL_OBJ_TYPE ('INDEXED_FACT_3','NUMBER','10');
              
  lo_headers_tbl(2).indexed_value_list.EXTEND;
  lo_headers_tbl(2).indexed_value_list(3) := DETAIL_OBJ_TYPE ('INDEXED_FACT_6','DATE','26-MAY-2016');
  
end;

When the above anonymous PL/SQL block is run, it gives the following error:

ORA-06530: Reference to uninitialized composite

ORA-06512: at line 10

I assume this relates to the initialisation of the lo_headers_tbl, but as you can see at line 6, this has already been initialised.

I would appreciate any pointers as to what I'm doing wrong.  Thanks.