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

$result = mysql_query("select email,password from membertb where email='$email' and password='$password'") ;

$num = mysql_num_rows($result) ;
print $num;
if($num<=0) {
echo "<br><br><center><font size='3' face='MS Sans Serif'><b>รหัสผ่านไม่ถูกต้องครับ</b></font></center>" ;
//print "<meta http-equiv=refresh content=2;URL=index.php>";
exit() ;
}

else {
$dbarr = mysql_fetch_array($result) ;

if($email!=$dbarr['email'] and $password!=$dbarr['password']) {
echo "รหัสผ่านไม่ถูกต้อง" ;
exit() ;
}

else {
$login_true = $email ;
session_register("login_true") ;
echo "<meta http-equiv='refresh' content='0 ;url=member_detail.php'>" ;
exit() ;
}
}