Window function trouble

I have a table like this:
 CREATE TABLE tbl (   host text NOT NULL,   adr ip4 NOT NULL,   usr text NOT NULL );

(ip4 is from the ip4r contrib module)
and I want the number of entries per address and per user:
 SELECT adr, usr, count(*) FROM tbl WHERE host = ?   AND adr <<= ? GROUP BY adr, usr ORDER BY adr, usr

That's pretty basic stuff and returns something like this:
 adr1 usr1_1 cnt1_1 adr1 usr1_2 cnt1_2 adr1 usr1_3 cnt1_3 adr2 usr2_1 cnt2_1 ...

But I want the address to be NULL if it's the same as the address of
the previous row. I came up with this:
 SELECT CASE lag(adr) OVER (ORDER BY adr)        WHEN adr THEN NULL        ELSE adr        END AS myaddr,        usr,
count(*)FROM tbl WHERE host = ?   AND adr <<= ? GROUP BY adr, usr ORDER BY adr, usr
 

This returns something like
 adr1 usr1_1 cnt1_1 NULL usr1_2 cnt1_2 NULL usr1_3 cnt1_3 adr2 usr2_1 cnt2_1 ...

what's exactly what I want.  But when I don't name the CASE expression
(i.e. I delete "AS myaddr"), I get the following:
 adr1 usr1_1 cnt1_1 adr2 usr2_1 cnt2_1 ...

The other users for one address are gone.  Does anyone know why?