Updating a table which is null doesn't work?

Create a table as follows:

    create table test_tbl (name varchar(3), item char(1)[])

Insert a row:

    insert into test_tbl values ('ABC',null);

Update a value in the array:

    update test_tbl set item[1] = 'Z';

Now do a select:

    select * from test_tbl;

What happened to the update?

It appears to be necessary to set the array to some non-null value
before it is possible to update any element in it.  This would appear
to be inconsistent with what happens when you update a non-arrayed column
that currently contains a null.
--
Mike Nolan