在我的情况下简单的PHP dom解析器奇怪的错误

Question

error msg : http://i.imgur.com/4UZIlks.png

I try to scrape the content post of this forum https://forum.lowyat.net/topic/3424996 using below code.

$rows = $html->find('.post_table');
$array = array();
foreach($rows as $go){
    $post_text = $go->find('.post_td_right > .post_text')->innertext;
    $array[]= array(
        'content'=> $post_text
    );
}

echo json_encode($array);

The loop isn't consistent I wonder why. I can get some result but there's an error saying Trying to get property of non-object, why?