ERROR: Bad interval external representation '0'

Hi

Is there a problem with averaging time intervals? I get this error with
AVG( inserttime - last)

ERROR:  Bad interval external representation '0'

but SUM( inserttime - last) / COUNT (inserttime) seems to work just fine.

Details below, database version is 7.1.

thanks,
Philip


sceppdata=> \d data_chunk
                    Table "data_chunk"
   Attribute   |           Type           |   Modifier
---------------+--------------------------+---------------
 chunkid       | integer                  | not null
 seq_num       | integer                  |
 channel_dbid  | integer                  |
 first         | timestamp with time zone |
 first_nanos   | integer                  |
 first_leapsec | integer                  |
 last          | timestamp with time zone |
 last_nanos    | integer                  |
 last_leapsec  | integer                  |
 inserttime    | timestamp with time zone | default now()
Indices: data_chunk__first_idx,
         data_chunk__last_idx,
         data_chunk__seq_num_idx,
         data_chunk_pkey

sceppdata=> SELECT AVG( inserttime - last) AS latency
 FROM data_chunk WHERE channel_dbid = 1 AND
last > '2001-06-15 10:35:54.85-04';
ERROR:  Bad interval external representation '0'
sceppdata=> SELECT SUM( inserttime - last) / COUNT (inserttime) AS latency
 FROM data_chunk WHERE channel_dbid = 1 AND
last > '2001-06-15 10:35:54.85-04';
   latency
-------------
 00:00:07.07
(1 row)

sceppdata=> SELECT inserttime, last, (inserttime - last) AS latency
sceppdata->  FROM data_chunk WHERE channel_dbid = 1 AND
last > '2001-06-15 10:35:54.85-04';
       inserttime       |           last            |   latency
------------------------+---------------------------+-------------
 2001-06-15 10:37:38-04 | 2001-06-15 10:37:29.70-04 | 00:00:08.30
 2001-06-15 10:39:36-04 | 2001-06-15 10:39:30.16-04 | 00:00:05.84
(2 rows)