I look for HttpExceptions in the Application_Error sub of my global.asx

Sub Application_Error(ByVal sender As Object, ByVal e As EventArgs)

        Dim ex As Exception = HttpContext.Current.Server.GetLastError()

        If ex IsNot Nothing Then
            If TypeOf (ex) Is HttpUnhandledException Then
                If ex.InnerException Is Nothing Then
                    Server.Transfer("error.aspx", False)
                End If
                ex = ex.InnerException
            End If

            If TypeOf (ex) Is HttpException Then
                Dim httpCode As Integer = CType(ex, HttpException).GetHttpCode()
                If httpCode = 404 Then
                    Server.ClearError()
                    Server.Transfer("error_404.aspx", False)
                End If
            End If
        End If
End Sub

I can step through this code and confirm it does hit the Server.Transfer("error_404.aspx"), as well as the Page_Load of error_404.aspx, but all it shows is a blank page.