Warning: mysql_num_rows() expects parameter 1 to be resource

if(mysql_num_rows($checklogin) == 1)

$checklogin = mysql_query("SELECT * FROM users WHERE Username = '".$username."' AND Password = '".$password."'");

    if(mysql_num_rows($checklogin) == 1)
    {
        $row = mysql_fetch_array($checklogin);
        $email = $row['EmailAddress'];

        $_SESSION['Username'] = $username;
        $_SESSION['EmailAddress'] = $email;
        $_SESSION['LoggedIn'] = 1;

        display_user_profile();
    }