php问题。。。

$query="select * from liuyan where bookseries='$book_series' order by id";
$result=mysql_query($query);
$num=mysql_num_rows($result); //这一行告警

总是显示这样的告警。。。

Warning: Supplied argument is not a valid MySQL result resource in /usr/local/apache/htdocs/gdgl/library/showbookdetail.php3 on line 89

谢谢！