REPOST: Trouble with SQL conversion

I am trying to write portable sql which will work on multiple
databases,
but am having trouble converting this query to postgres 7.1.3:

SELECT base.baseindex, base.subnetsize, base.descrip, base.baseaddr,
base.admingrp, count(ipaddr.baseindex) AS cnt FROM base
LEFT JOIN ipaddr ON base.baseindex=ipaddr.baseindex WHERE
base.baseaddr
BETWEEN 2473473024 AND 2473473151 AND base.customer=1
GROUP BY base.baseindex
ORDER BY base.baseaddr;

I get this error:

0: ERROR: Attribute base.subnetsize must be GROUPed or used in an
aggregate function

The table structures are:

CREATE TABLE base ( baseaddr bigint NOT NULL default '0', subnetsize int NOT NULL
default '0', descrip varchar(80) NOT NULL default '', baseindex serial, admingrp varchar(40) NOT NULL default '',
customersmallint NOT NULL default '0', lastmod timestamp, userid varchar(40) NOT NULL default '', swipmod timestamp,
PRIMARYKEY  (baseindex), UNIQUE (baseaddr,customer)
 
);
CREATE INDEX base_customer ON base (customer); CREATE INDEX
base_admingrp
ON base (admingrp);

CREATE TABLE ipaddr ( ipaddr bigint NOT NULL default '0', userinf varchar(80) NOT NULL default '', location varchar(80)
NOT
NULL default '', telno varchar(15) NOT NULL default '', descrip
varchar(80) NOT NULL default '', baseindex int NOT NULL default '0', lastmod timestamp NOT NULL, userid varchar(40) NOT
NULLdefault '', PRIMARY KEY 
 
(ipaddr,baseindex)
);
CREATE INDEX ipaddr_baseindex ON ipaddr (baseindex);