detecting a NULL box

Does anybody know how to detect a NULL in a geometric box type?

When I execute the following sql statement (coords is a box type)
autotest=# select coords from dlg_control limit 1
autotest-# \g
coords
--------

(1 row)

So, I have data that contains a "NULL" in the coords column, but when I
test on box(NULL) I don't get any data...
autotest=# select coords from dlg_control where coords=box(NULL) limit 1
autotest-# \g
coords
--------
(0 rows)

Any ideas?
Thanks,
Dennis