Extend the sitemap protocol with custom element

I'm trying to extend the sitemap.xml protocol (per the sitemaps.org website's instructions), but the XML document fails validation, giving an error "rid element is not declared".

I've set the namespace on the "rid" element, but can't figure out what I'm overlooking.

XML Document

<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://ift.tt/xwbjRF"
        xmlns:xsi="http://ift.tt/ra1lAU"
        xmlns:custom="http://ift.tt/1tnKvlK"
        xsi:schemaLocation="http://ift.tt/xwbjRF
                            http://ift.tt/Av6BHN
                            http://ift.tt/1tnKvlK 
                            http://ift.tt/1tnKvlM">
  <url>
    <loc>http://ift.tt/1tnKvlQ;
    <lastmod>2013-04-24</lastmod>
    <priority>1.0</priority>
    <custom:rid>131</custom:rid>
  </url>
</urlset>

XSD Document

<?xml version="1.0"?>
<xs:schema elementFormDefault="qualified" 
           targetNamespace="http://ift.tt/1tnKvlK"
           xmlns:xs="http://ift.tt/tphNwY">
  <xs:element name="rid" type="xs:int"/>
</xs:schema>

Any help would be appreciated.

(Related Extending a sitemap)