Custom Field to Magpie Problem

Hi Folks,

I have some external RSS feeds that I need to include on various pages about people.

In my people entries I include firstname lastname. So made sure the RSS feeds are named that way as well. I tried the below but nothing.

{exp:magpie url="siteurl.com/{firstname}-{lastname}.rss" limit="10" refresh="720"}
<ul>
{magpie:items}
<li><a href="http://{magpie:link}">{magpie:description}</a> - {magpie:title}</li>
{/magpie:items}
</ul>
{/exp:magpie}

If I put in the actual url of the feed it works and those custom fields output the correct info. Is it because the two are examined at the same time? How would I get round that?

Thanks James