pg_freespacemap question

Hi,

I tried pg_freespacemap and found strange result:

test=# select * from pg_freespacemap where blockfreebytes = 0;
 blockid | relfilenode | reltablespace | reldatabase | relblocknumber | blockfreebytes 
---------+-------------+---------------+-------------+----------------+----------------
      25 |        2619 |          1663 |       16403 |              0 |              0
      63 |        2619 |          1663 |       16384 |             10 |              0
(2 rows)

Is it possible that a free space map entry has 0 blockfreebytes?
--
Tatsuo Ishii
SRA OSS, Inc. Japan