Ma perchè non mi fa questa divisione, mentre la sottrazione si ?

[The knight]

$list = $db->fetch("SELECT " . JPREFIX . "sites.id, name, domain, address, " . JPREFIX . "list.in_hits as rin, " . JPREFIX . "list.out_hits as rout, " . JPREFIX . "archive.in_hits as ain, " . JPREFIX . "archive.out_hits as aout FROM " . JPREFIX . "sites LEFT JOIN " . JAJPREFIX . "list ON " . JPREFIX . "list.id=" . JPREFIX . "sites.id LEFT JOIN " . JPREFIX . "archive ON " . JPREFIX . "list.id=" . JPREFIX . "archive.id WHERE status=1 ORDER BY " . JPREFIX . "list.in_hits DESC");
foreach($list as $website) {
    $website["rin"] = ($website["rin"]=="") ? 0 : $website["rin"];
    $website["rout"] = ($website["rout"]=="") ? 0 : $website["rout"];
    $website["ain"] = ($website["ain"]+$website["rin"]=="") ? 0 : $website["ain"]+$website["rin"];
    $website["aout"] = ($website["aout"]+$website["rout"]=="") ? 0 : $website["aout"]+$website["rout"];
    $website["credits"] = $website["ain"] / $website["aout"]; 

Mi dà questo errore "Warning: Division by zero in "