Codeigniter致命错误在布尔值上调用成员函数result（）

I have been having a problem with a query which results in the following error

PHP Fatal error:  Call to a member function result() on boolean on line 200

I'v had no problems with other queries, but on this one I have to do a query within a query, which is the following:

$this->load->helper('date');
$datestring = "%Y-%m-%d";
$time = strtotime('yesterday');
$date= mdate($datestring, $time);

$db = $this->load->database('macs', TRUE);

$query = $db->query("SELECT TEMP1.USER AS PACKER, Count(TEMP1.[ORDER]) AS ORDERS FROM (SELECT AUDIT.USER, AUDIT.[ORDER] FROM AUDIT WHERE AUDIT.DATE > '$date' AND (AUDIT.TYPE='I11' Or AUDIT.TYPE =' 11') GROUP BY AUDIT.DATE, AUDIT.TYPE, AUDIT.USER, AUDIT.[ORDER]) AS TEMP1 GROUP BY TEMP1.USER ORDER BY Count(TEMP1.[ORDER]) DESC");

return $query->result();
$macsdb->close();

This works correctly in access (The above is a conversion of an access query). The query starts to work (incorrectly however) when anything referencing USERis removed and even works when USER is replaced with another field such as Company. What could be causing this query to not be working with USER?