Try guestbook in lower case. For whatever reason, I haven't quite figured out why, but this fixed it for me. I know for a fact that in my db it has capitalized letters (I camelCased it), but I had to enter the datasource in all lower case.
Hope this helps,
© 2014, O’Reilly Media, Inc.
All trademarks and registered trademarks appearing on oreilly.com are the property of their respective owners.