Error of query and insertion in php

My problem just happened to me, my code was quite good. The detail was that I modified a couple of things in Conect_Db and after that when I try to enter my data I see this error in my PHP code line, someone who can you help me? ...

  

Warning: mysql_num_rows () expects parameter 1 to be resource, boolean given in

Code:

<?php

    $realname=$_POST['realname'];
    $mail=$_POST['nick'];
    $pass= $_POST['pass'];
    $rpass=$_POST['rpass'];

    require("connect_db.php");
    $checkemail=mysql_query("SELECT * FROM login WHERE email='$mail'");
    $check_mail=mysql_num_rows($checkemail)or die(mysql_error());//or die (mysql_error()) -- se uso la sentencia para cerrar la comprobacion del check mail
        if($pass==$rpass){
            if($check_mail>0){
                echo ' <script language="javascript">alert("Atencion, ya existe el mail designado para un usuario, verifique sus datos");</script> ';
            }else{

                //require("connect_db.php");
                mysql_query("INSERT INTO login VALUES('','$realname','$pass','$mail','')")or die(mysql_error());
                //echo 'Se ha registrado con exito';
                echo ' <script language="javascript">alert("Usuario registrado con éxito");</script> ';
                mysql_close($link);
            }

        }else{
            echo 'Las contraseñas son incorrectas';
        }


?>