Google Rich Snippets: Warning: Please provide best and worst.

If you're getting the message "Warning: Please provide best and worst." in the Google Rich Snippets testing tool, it is likely that one of your fields is 0. In my case it was the 'best' field that needed to be set to 5 and the 'average' field was 0, but Google Rich Snippets needs your average to be at least 1 it seems (indicating that your item has actually had some reviews).