Google XML Sitemaps generating incorrect URLs

The plugin is creating URLs that look like this:

http://993c4s.com//nascars-silent-partner/

The problem is that I have my permalinks set to use category and post name

so the real URL is

http://993c4s.com/porsche-products/porsche-books-resources-porsche-culture/nascars-silent-partner/

Any idea why it would generate the incorrect URL?