ORA-01461 (with > 4k varchar2) error Only in merge statement. Insert or update works fine

Here is my clue... I'm on oracle 11g. Searched a lot, but nothing found.

I need to execute DML operations, which can contain data > 4k characters.

If i use a sql block, directly in oracle, like the next one, everything works fine

declare 
  txtV varchar2(32000);
BEGIN 
  txtV:= 'MORE THAN 4k CHARS, here only few for readability' ; 
  Update FD_FILTERDEF 
     set SQLFILTER = txtV 
   where id='blabla';  
END;

BUT!!! if i use merge statement, it gives me error ORA-01461

declare 
  txtV varchar2(32000);
BEGIN 
  txtV:= '' ; 
  MERGE INTO FD_FILTERDEF A 
        USING ( select  txtV C0 
                  from dual) ST 
           ON (A.CODE = 'bla bla') 
  WHEN MATCHED THEN 
    Update set A.SQLFILTER = st.C0  
  WHEN NOT MATCHED THEN 
    insert (CODE  ,SQLFILTER ) 
      values ('bla bla'  , ST.C0  );  
END; 

If have some hint would be appreciated :)

Use this:

create table fd_filterdef
( code varchar2(10) primary key
, sqlfilter clob );

declare
    txtv varchar2(32000);
begin
    txtv := rpad('select statement, really really long', 5000, ' etc');

    merge into fd_filterdef a
    using (select 'bla bla' as code from dual) st
    on (a.code = st.code)
    when matched then
        update set a.sqlfilter = txtv
    when not matched then
        insert (code, sqlfilter)
        values (st.code,txtv);
end;
/

select code, length(sqlfilter) from fd_filterdef;

CODE       LENGTH(SQLFILTER)
---------- -----------------
bla bla                 5000

Selecting your long variable from dual implicitly casts it to a SQL varchar2 which prior to 12c only holds up to 4000 bytes.