`gitbookInfo()` produces error

I encountered the following error when trying to run `gitbookInfo()` as described in your [online tutorial](http://jason.bryer.org/Rgitbook/installation/gitbookInfo.html). 

```
> gitbookInfo()
npm http GET https://registry.npmjs.org/gitbook
npm http 304 https://registry.npmjs.org/gitbook
Error in if (current == installed) { : argument is of length zero
```

Do you know where this comes from?
