PDO错误 - PDOException',消息'SQLSTATE [HY000]:常规错误'

我收到此错误:

Fatal error: Uncaught exception 'PDOException' with message 'SQLSTATE[HY000]: General error' in ...

..每当我用PDO执行此代码时:

//Select data from the topic.
$s = $dbh->prepare("SELECT * FROM forum_topics WHERE forum_id=:forum_cat_id AND topic_id=:topicid");
$s->bindParam(':forum_cat_id', $forum_cat_id);
$s->bindParam(':topicid', $topicid);
$s->execute();
$f = $s->fetch();

$s = $dbh->prepare("UPDATE forum_cats 
    SET 
        forum_last_postid = :last_post_id, forum_last_posttime = :time, 
        forum_last_userid = :userid, forum_last_username = :username, 
        forum_posts=forum_posts+1 
    WHERE forum_id = :forum_cat_id");
$s->bindParam(':last_post_id', $last_post_id);
$s->bindParam(':time', $time);
$s->bindParam(':userid', $userid);
$s->bindParam(':username', $userdata['username']);
$s->bindParam(':forum_cat_id', $forum_cat_id);
try {
    $s->execute();
}
catch(PDOException $e) {
    die($e->getMessage());
}

if (count($s->fetchAll()) == 0) {
    return 3;
}

我不知道为什么会这样.我检查了查询,我根本找不到任何错误..