This is the second site that I have set up recently with this problem. If a file is not found it doesn't give a 404 it gives a 500 error. This is a problem when google visits the site and looks for robots.txt if you don't have a file by that name. It stops google from indexing the site. I have had to set up an empty file to get around it. Here's a link to see what you get for a file not found http://www.firstchoiceridingacademy.com/robts.txt. (I intentionally spelled robots wrong since I have now created an empty robots.txt file)

Then it seems to resolve itself at some point since another of my sites now gives the 404.