Warning: mysql_fetch_array(): supplied argument is not a valid MySQL result resource

[lobbys]

ayudaaaaaaaaaaaaaaaaaaaaaaaaa
me sale este error
Warning: mysql_fetch_array(): supplied argument is not a valid MySQL result resource in /home/osaed/public_html/tutoria/include/db.php on line 28

que me falta????????????
este es mi codigo php:

<?
define('DB_SERVER', 'localhost');
define('USER_DB', 'osaed_tutoria');
define('PASSWORD_DB', 'osaeduigv2007');
define('DATABASE', 'osaed_tutoria');

function db_connect($server = DB_SERVER, $user = USER_DB, $password = PASSWORD_DB, $database = DATABASE, $link = 'link_db'){
global $$link;

$$link = mysql_connect($server, $user, $password);

if($$link) mysql_select_db($database);

return $$link;
}

function db_query($query, $link = 'link_db'){
global $$link;
//echo $query.'<br>';
//$result = mysql_query($query, $$link);
$result = mysql_query($query) or die ("Invalid query");

return $result;

}
function db_fetch_array($query){
//echo 'aaaaa'.$query;
return mysql_fetch_assoc($query);
}

function db_num_rows($query){
return mysql_num_rows($query);
}

function db_insert_id(){
return mysql_insert_id();
}

?>