SQL & PL/SQL

DB version EE 9.2.0.6 on Win2K.

I get error "character string buffer too small" when running procedure in a package. Simplified (there are actually more parameters in the procedure call):

procedure x(p_var   in out varchar2)
  v_local   varchar2(4000)
begin
  v_local := 'a very long string .....';
  p_var := v_local;    -- Line giving error
end;

It works OK if I limit the output to 1000 characters (1001 is already too long):

procedure x(p_var   in out varchar2)
  v_local   varchar2(4000)
begin
  v_local := 'a very long string .....';
  p_var := substr(v_local,1,1000);
end;

I never heard of a limit of 1000 characters for a varchar2 procedure parameter, though. Any ideas?