SQL & PL/SQL

Hi everyone.
i got a table with a column which type is number.
the idea is to store there a boolean value (0/1)
i store the values with a function with a boolean parameter. i, then, converse the boolean with diutil.bool_to_int().
however when i try to compile, i get the PLS-00382: expression is of wrong type and i don't know why.

As example:

create table valores
(id number,
boolean_value number);

alter table valores
add constraint SSP_CK5
check (boolean_value IN (0,1));

create or replace function intro
(p_id in valores.id%type,
p_valor in boolean default true)
return boolean is
begin
insert into valores (id, boolean_value)
values (p_id, sys.diutil.bool_to_int(p_valor));
commit work;
return true;
end intro;

By the way, i'm working in 9i.

Regards.-