Rjb-1.3.7 is for Ruby-1.9.x (no functionally change was made for Ruby-1.8.x) that correct re-raising Rjb's exception.
Ruby 1.9 re-raise makes an outer exception (RuntimeError) for the original exception. And at that time it calls to_str method and the previous release of Rjb's exception has no definition and causes another error.
This problem was reported by Joeri Samson, thanks Joeri.