Why Does Custom HttpResponse Throw Exception On HttpResponse.End()

I'm writing some code in which I need to use my own HttpResponse object to capture the output from a method on another object that takes an HttpResponse as a parameter. The problem is, this other object (which I cannot modify) calls HttpResponse.End(), which causes an "Object reference not set to an instance of an object" exception to be thrown. What can I do about this?

Dim myStringbuilder As New StringBuilder
Dim myStringWriter As New IO.StringWriter(myStringbuilder)
Dim myResponse As New Web.HttpResponse(myStringWriter)

someObject.doStuffWithHttpResponse(myResponse) ' calls myResponse.End() and crashes

Here's some more complete information about the error, thrown from the following code in a console application:

Dim myStringbuilder As New StringBuilder
Dim myStringWriter As New IO.StringWriter(myStringbuilder)
Dim myResponse As New Web.HttpResponse(myStringWriter)
Try
myResponse.End()
Catch ex As Exception
Console.WriteLine(ex.ToString)
End Try

Here's the text of the exception:

System.NullReferenceException: Object reference not set to an instance of an object.
at System.Web.HttpResponse.End()
at ConsoleApplication1.Module1.Main() in C:Documents and Settingsjoe.userLocal SettingsApplication DataTemporary ProjectsConsoleApplication1Module1.vb:line 10