SQL Developer

Hi,
Is there any reason why I would not be able to store more than 999 characters in a variable of varchar2 type?
I keep getting error italicsORA-06502:PL/SQL: numeric or value error:character string buffer too smal+italics+ when run the following code in PL/SQL Developer 8.0.4.1514:

declare
st varchar2(9999 CHAR) := 'asdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasdasd';
begin
DBMS_OUTPUT.enable(200000);
--st := st || 'aa';
dbms_output.put_line(st);
end;

I built the code above just to demonstrate that PL/SQL refuses to work with more than 999 characters. The initial string contains 999 characters, and I can run the code. If I uncomment concatenation of 'aa' then I get the error.
I can't find any reason why. Documentation says that I can store up to 32000 characters. Does anyone know why it's happening or what I'm doing wrong?