C# - Visual Studio 2008 Web Application Keeps Throwing Error When First Run It?

As stated in the title my web application builds successfully, although every time I run it in debug mode I get the following .Net error:

If I hit refresh then the application gets no more errors until I next start it up again, any ideas?

Here is my global.asax file:

<%@ Application Language="C#" Inherits="MyCompany.Web.MyApp.Shell.CustomWebClientApplication" %>
<script runat="server">
void Session_End(Object Sender, EventArgs e)
{
FormsAuthentication.SignOut();
}
protected void Session_Start(Object sender, EventArgs e)
{
if (Session.IsNewSession)
{
Response.Redirect(System.Web.Configuration.WebConfigurationManager.AppSettings["HomePage"]);
}
}
protected void Application_Error(Object sender, EventArgs e)
{
System.Exception oops = Server.GetLastError();
//Injection attack error handling
if (oops.GetBaseException() is System.Web.HttpRequestValidationException)
{
Response.Redirect("ErrorPage.aspx");
}
}
</script>