Unable to read value from Web.Config

I'm trying to implement a recaptcha on a login page. But every time it hits the code , a null reference exception happens. I'm using asp.net with the default login template, it has its own Web.Config(not sure why though)

Is there a setting that needs to be done?Below are my codes.Hope to get a reply soon.

aspx file

<%@ Page Title="Log In" Language="C#" MasterPageFile="~/Site.master" AutoEventWireup="true"CodeBehind="Login.aspx.cs" Inherits="Penerangan.Emedia2u.Web.Account.Login" %>
<%@ Register TagPrefix="recaptcha" Namespace="Recaptcha" Assembly="Recaptcha" %>

<asp:Label Visible="false" ID="lblResult" runat="server" />
                    <recaptcha:RecaptchaControl ID="recaptcha" runat="server" />

.cs file

  recaptcha.PublicKey = System.Configuration.ConfigurationManager.AppSettings["reCAPTCHAPublicKey"].ToString();
                recaptcha.PrivateKey = System.Configuration.ConfigurationManager.AppSettings["reCAPTCHAPrivateKey"].ToString(); 

web.config file

 <appSettings>
    <add key="reCAPTCHAPublicKey" value="fromgooglecaptchakey"/>
    <add key="reCAPTCHAPrivateKey" value="fromgooglecaptchakey"/>
  </appSettings>