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

Hi,
I keep getting this error message when i run register.php:
Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource in /home/httpd/vhosts/tellproperties.com/httpdocs/testing/register.php on line 42

Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource in /home/httpd/vhosts/tellproperties.com/httpdocs/testing/register.php on line 43
No Database Selected

Register.php
[code]
<?
include 'listing.php';
// Define post fields into simple variables
$first_name = $_POST['first_name'];
$last_name = $_POST['last_name'];
$email_address = $_POST['email_address'];
$username = $_POST['username'];
$info = $_POST['info'];
/* Let's strip some slashes in case the user entered
any escaped characters. */
$first_name = stripslashes($first_name);
$last_name = stripslashes($last_name);
$email_address = stripslashes($email_address);
$username = stripslashes($username);
$info = stripslashes($info);
/* Do some error checking on the form posted fields */
if((!$first_name) || (!$last_name) || (!$email_address) || (!$username)){
    echo 'You did not submit the following required information! <br />';
    if(!$first_name){
        echo "First Name is a required field. Please enter it below.<br />";
    }
    if(!$last_name){
        echo "Last Name is a required field. Please enter it below.<br />";
    }
    if(!$email_address){
        echo "Email Address is a required field. Please enter it below.<br />";
    }
    if(!$username){
        echo "Desired Username is a required field. Please enter it below.<br />";
    }
    include 'advertise.php'; // Show the form again!
    /* End the error checking and if everything is ok, we'll move on to
     creating the user account */
    exit(); // if the error checking has failed, we'll exit the script!
}
/* Let's do some checking and ensure that the user's email address or username
does not exist in the database */
 $sql_email_check = mysql_query("SELECT email_address FROM users  
            WHERE email_address=\"$email_address\"");
 $sql_username_check = mysql_query("SELECT username FROM users  
            WHERE username=\"$username\"");
 $email_check = mysql_num_rows($sql_email_check);
 $username_check = mysql_num_rows($sql_username_check);
 if(($email_check > 0) || ($username_check > 0)){
    echo "Please fix the following errors: <br />";
    if($email_check > 0){
        echo "<strong>Your email address has already been used by another member
        in our database. Please submit a different Email address!<br />";
        unset($email_address);
    }
    if($username_check > 0){
        echo "The username you have selected has already been used by another member
         in our database. Please choose a different Username!<br />";
        unset($username);
    }
    include 'advertise.php'; // Show the form again!
     exit();  // exit the script so that we do not create this account!
 }
/* Everything has passed both error checks that we have done.
It's time to create the account! */
/* Random Password generator.
http://www.phpfreaks.com/quickcode/Random_Password_Generator/56.php
We'll generate a random password for the
user and encrypt it, email it and then enter it into the db. */
function makeRandomPassword() {
  $salt = "abchefghjkmnpqrstuvwxyz0123456789";
  srand((double)microtime()*1000000);  
      $i = 0;
      while ($i <= 7) {
            $num = rand() % 33;
            $tmp = substr($salt, $num, 1);
            $pass = $pass . $tmp;
            $i++;
      }
      return $pass;
}
$random_password = makeRandomPassword();
$db_password = md5($random_password);
// Enter info into the Database.
$info2 = htmlspecialchars($info);
$sql = mysql_query("INSERT INTO users (first_name, last_name,
        email_address, username, password, info, signup_date)
        VALUES('$first_name', '$last_name', '$email_address',
        '$username', '$db_password', '$info2', now())")  
        or die (mysql_error());
if(!$sql){
    echo 'There has been an error creating your account. Please contact the webmaster.';
} else {
    $userid = mysql_insert_id();
    // Let's mail the user!
    $subject = "Your Membership at MyWebsite!";
    $message = "Dear $first_name $last_name,
    Thank you for registering at our website, http://www.mydomain.com!
    You are two steps away from logging in and accessing our exclusive members area.
    To activate your membership,
    please click here: http://www.mydomain.com/activate.php?id=$userid&code=$db_password
    Once you activate your memebership, you will be able to login
    with the following information:
    Username: $username
    Password: $random_password
    Thanks!
    The Webmaster
    This is an automated response, please do not reply!";  
    mail($email_address, $subject, $message,  
        "From: MyDomain Webmaster<admin@mydomain.com>\n
        X-Mailer: PHP/" . phpversion());
    echo 'Your membership information has been mailed to your email address!
    Please check it and follow the directions!';
}
?>
[code]

***********************
This is my listing.php connection file:

[code]
<?php
# FileName="Connection_php_mysql.htm"
# Type="MYSQL"
# HTTP="true"
$hostname_listing = "localhost";
$database_listing = "listing";
$username_listing = "admin";
$password_listing = "helpme";
$listing = mysql_pconnect($hostname_listing, $username_listing, $password_listing) or trigger_error(mysql_error(),E_USER_ERROR);
?>
[code]

I have tried commenting out the line 42 and 43 but still get no database selected.  Even if i delete the whole email and username check function i still get no database selected.

Any help much appreciated. Thanks in advance for your help.
MsKazza