Error in updating CONTEXT Index

hi,
we need to update the index created on a CLOB field after every insert,delete or update of data.
Code we used to create trigger is shown below:

CREATE TRIGGER scott.S_SEARCH_TRIGGER AFTER
INSERT
OR DELETE
OR UPDATE ON scott.S_SEARCH
BEGIN
dbms_utility.exec_ddl_statement('alter index scott.s_search.index rebuild online parameters(''sync memory 45M'')');
END;

Trigger creation is successful..but whenever we do an insert,update or delete the following error is showing up:

for ex: if we issue a query like insert into scott.s_search values(6,'ertet')

ERROR at line 1:
ORA-02243: invalid ALTER INDEX or ALTER MATERIALIZED VIEW option
ORA-06512: at "SYS.DBMS_SYS_SQL", line 826
ORA-06512: at "SYS.DBMS_SQL", line 32
ORA-06512: at "SYS.DBMS_UTILITY", line 550
ORA-06512: at "SCOTT.S_SEARCH_TRIGGER", line 2
ORA-04088: error during execution of trigger 'SCOTT.S_SEARCH_TRIGGER'

Can anybody tell y is this so? or does nebody have a better code to rebuild CONTEXT index?

any help would be highly appreciated..

Regards.

Got it...(this may be useful for beginners )
Instead of trigger, we created a procedure to update index and submitted that procedure via dbms_jobs.submit to be run in regular intervals.
Code we used to create procedure:

create or replace procedure scott.index_rebuild_procedure
is
begin
dbms_utility.exec_ddl_statement('alter index scott.pobox_index rebuild online parameters(''sync memory 45M'')');
end index_rebuild_procedure;

Submitted the above procedure using PL/SQL with an interval of ONE Minute as:

DECLARE
v_jobno number;
BEGIN
dbms_job.submit(v_jobno, scott.index_rebuild_procedure ;',sysdate,'sysdate + 1/1440');
commit;
END;