Database Software

Dear sirs,

I am a new user of Oracle Text (Oracle 11g release 11.2) and I am unable to create an index of type ctxsys.context). Any suggestions?:

code:
drop table mytable;
drop index myindex force;
create table mytable(id number primary key, docs clob);

insert into mytable values(111555,'this text will be indexed');
insert into mytable values(111556,'this is a default datastore example');
commit;

create index myindex on mytable(docs)
indextype is ctxsys.context
parameters ('DATASTORE CTXSYS.DEFAULT_DATASTORE');
+++++++
error messages:
create index myindex on mytable(docs)
*
ERROR at line 1:
ORA-29855: error occurred in the execution of ODCIINDEXCREATE routine
ORA-20000: Oracle Text error:
ORA-06508: PL/SQL: could not find program unit being called
ORA-06512: at "CTXSYS.DRUE", line 160
ORA-06512: at "CTXSYS.TEXTINDEXMETHODS", line 366