Virtus wrapper breaks: undefined method 'primitive'

When I try to use the Virtus wrapper, it falls down due to unimplemented method 'primitive'.

https://gist.github.com/anicholson/10c85789028d998a9087

Same error occurred when I cloned the repository and attempted to run the tests. Can you please fix this?
EDIT: Or help me work out what the problem is :)
