Errore mysql_fetch_assoc()

[iceberg]

Ciao a tutti, secondo voi da cosa dipende questo errore

mysql_fetch_assoc(): supplied argument is not a valid MySQL result resource in13

in questo codice

function get_prints($user) {
$conn = get_db_conn();

$res = mysql_query('SELECT `from`, `to`, `time` FROM footprints WHERE `to`=' . $user . ' ORDER BY `time` DESC', $conn);
$prints = array();
while ($row = mysql_fetch_assoc($res)) {
$prints[] = $row;
}
return $prints;

Grazie