problem with dbms_metadata.get_ddl in plsql

 Hi,
How can I used the dbms_metadata.get_ddl in pl/sql and insert the INDEX source in a table.I am using the following code but getting 'ORA-01422: exact fetch returns more than requested number of rows'


set serveroutput on
declare
 lv_object_type   varchar2(30);               
 lv_buffer_1        varchar(4000);
 lv_buffer_2        varchar(4000);
begin                                                 
  insert into  gtt_1  select to_char(dbms_metadata.get_ddl( 'MATERIALIZED_VIEW','COM_CUST_PROD_AGG_MV1','T709374')) from dual   ;             
  dbms_output.put_line(lv_buffer_1) ;     
  commit ;
end; 

when I run the following select on sqlplus :
select to_char(dbms_metadata.get_ddl( 'MATERIALIZED_VIEW','COM_CUST_PROD_AGG_MV1','T709374')) from dual   ;