title: How do you preview a proration using Stripe.NET?
body: <p>I'm following the example provided in the Stripe documentation on <a href="https://stackoverflow.com/questions/tagged/stripe-payments">Previewing Proration</a> using the Stripe.NET library to try to find the amount that will be charged when a customer upgrades from Plan A to Plan B.</p>
  
  <p>However, when I use the code sample in the documentation, I get an error:</p>
  
  <pre><code>UpcomingInvoiceOptions options = new UpcomingInvoiceOptions()
  {
      CustomerId = "cus_XXXXXXXXXXXXX",
      SubscriptionProrationDate = DateTime.UtcNow,
      SubscriptionItems = new List&lt;InvoiceSubscriptionItemOptions&gt;()
      {
          new InvoiceSubscriptionItemOptions()
          {
              Id = "si_XXXXXXXXXXXXXX", // Current Sub Item
              PlanId = "plan_XXXXXXXXXXXX" // New plan
          }
      }
  };
  InvoiceService service = new InvoiceService();
  var result = service.Upcoming(options);
  </code></pre>
  
  <p>The last line throws a <strong>Stripe.StripeException</strong>: <code>You cannot update a subscription item without a subscription.</code></p>