Getting error when trying to build book with newest version of bookdown

When I update bookdown from 0.21 to 0.22, I am no longer able to build the book and get the following error:
```
Error in setwd(book_proj) : character argument expected
Calls: <Anonymous> -> site_generator -> create_site_generator -> setwd
Execution halted

Exited with status 1.
```
I also tried to dev version (0.22.2) and the error persists.