Object Reference Null - URL Rewriting

I have developed my asp.net website in .NET 2.0 in other system where it is working fine. Now when I copied the asp.net website in my system and run it than I am getting the run time error:

Object reference not set to an
instance of an object.
public class FixURLs : IHttpModule
{
public FixURLs()
{
}
#region IHttpModule Members
public void Dispose()
{
// do nothing
}
public void Init(HttpApplication context)
{
context.BeginRequest += new EventHandler(context_BeginRequest);
context.CompleteRequest();
}

some other logic. I am getting object reference error at the line: context.CompleteRequest(); My web.Config file has

<compilation debug="true">
<assemblies>
<add assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
</assemblies>
</compilation>......