C# UrlReferrer from PayPal return to vendor button? Object Not Set an instance of an object

Question

After a successful Paypal transaction my customer lands on a Paypal Thank You page with a link back to my site. That link is going to the expected page. However If I attempt to pull Referrer information from my server code, I get error

Object Not set to an Instance of an object.

I have a page on the site with a link to that same page and it successfully returns the URL calling it.

I hover over the Paypal return to vendor button and it's just a link to my page.

Here's my code.

Uri MyUrl = Request.UrlReferrer;
Response.Write(MyUrl.AbsoluteUri.ToString());

Could this be an issue with the page already havng been called. I've refressed the calling and destination pages. Just want to avoid re-entering CC info so I'm using the back button. Using back button and then refreshing I think should work.

Thanks.