SQL & PL/SQL

I am new to Oracle:

I have simple function Is_Empty for checking length of a variable:

create or replace function Is_Empty(p in VARCHAR2) return boolean
as
begin
if length(p) = 0 then
return true;
else
return false;
end if;
end;


select is_empty('xxx') from dual;

when it ran .. I got the following error message:

ORA-06552: PL/SQL: Statement ignored ORA-06553: PLS-382: expression is of wrong type

What I missed?

please help

thanks