The gem is breaking on the "__method__" method under the Kernel module on line 130 of the extensions.rb file.

The current declaration (which breaks) is:

def __method__(depth = 0)
  caller[depth][/`([^']+)'/, 1]
end if RUBY_VERSION < '1.8.7'

and when changing RUBY_VERSION < '1.8.7' to RUBY_VERSION <= '1.8.7' it works fine and all tests pass.

Before:
146 tests, 314 assertions, 9 failures, 39 errors

After:
146 tests, 441 assertions, 0 failures, 0 errors