SQL & PL/SQL

Hi, can you tell me the reason for this error??

CREATE TABLE provatab1(
id_doc NUMBER(5) PRIMARY KEY,
nome_doc VARCHAR2(200),
fmt_doc VARCHAR2(10),
language VARCHAR2(20) DEFAULT 'italian'
);


Every document in a table contains 10 pages, written in italian language.

INSERT INTO provatab1 VALUES(1,'RapportiSiciliaMagnaGreciaEtruriaTestimonianzaAraTarquinia.doc','binary',default)

INSERT INTO provatab1 VALUES(2,'CeramicheGrecheTarquinia.doc','binary', default);

begin
ctx_ddl.create_preference('WITHPATH','FILE_DATASTORE');
ctx_ddl.set_attribute('WITHPATH','PATH','D:\docs tesi');
end

create index provaindex1 on provatab1 ( nome_doc )
indextype is ctxsys.context
parameters ( 'DATASTORE withpath filter ctxsys.inso_filter section group ctxsys.html_section_group' );



create global temporary table prova1
(
query_id number constraint prova1 primary key,
document varchar2(4001)
) on commit preserve rows;

Begin
Ctx_Doc.Set_Key_Type ( Ctx_Doc.Type_Primary_Key );
Ctx_Doc.Filter
(
index_name => 'provaindex1',
textkey => '1',
restab => 'prova1',
query_id => 1,
plaintext => true /* try the effect of FALSE */
);
end;

When I write:
select * from prova1;

the error is:
ORA-06502: PL/SQL: numeric or value error: character string buffer too small.

Why??????

Thank you!