รบกวนดูโค๊ดให้หน่อยครับ ผิดตรงไหน Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource in

<?
$user_login=$_POST[user_login];
$pass_login=$_POST[pass_login];
if ($user_login=="" or $pass_login==""){
	echo "<h3> Error : Please fill username or password </h3>"; exit();
}
include "connect.php";
$sql="select * from tb_member where username='$user_login' and password='$pass_login'";
$result=mysql_db_query($dbname,$sql);
$num=mysql_num_rows($result);
mysql_close();
if($num<=0){
	echo "<h3> : Invalid username or password </h3>";
}else {
session_start();
$_SESSION[sess_userid]=session_id();
$_SESSION[sess_username]=$user_login;
header ("Location : main.php");	
}
?>