bug on CHAR_LENGTH function ?

Hi all,

Seems that CHAR_LENGTH function has a bug.
It doesn't work if you try to have the length of columns with nulls.
Example:

insert into c values('foo');
insert into c values(NULL);
insert into c values('');

select b from c;
b
-------
foo
            <-- this is a NULL
            <-- this is a ''
(3 rows)

select char_length(b) from c;
ERROR:  Bad (null) varchar() external representation

-- Why this error?
-- I think it should be:

length
------
     3
        <-- length of NULL should be "unknown" this is the behavior of
other databases
     0
(3 rows)

What do you think about ?
       Jose'