Error Description

The gem seems incompatible with faker v1.9.0+. With the following versions defined in the Gemfile:

gem 'faker',          '1.9.1'
gem 'faker_japanese', '0.2.4'

loading the application fails with the following error:

/usr/local/rvm/gems/ruby-2.5.1/gems/bundler-1.16.3/lib/bundler/runtime.rb:84:in `rescue in block (2 levels) in require': There was an error while trying to load the
 gem 'faker_japanese'. (Bundler::GemRequireError)
Gem Load Error is: wrong number of arguments (given 1, expected 0)
Backtrace for gem load error is:
/usr/local/rvm/gems/ruby-2.5.1/gems/faker_japanese-0.2.4/lib/faker_japanese.rb:27:in `initialize'

Cause

This seems due to a new Faker::String class created in version 1.9.0 of faker:

faker-ruby/faker@v1.8.7...v1.9.0#diff-606ca7ead800c03c3025f08d2afca96d

And by chance, the Kanji class in the lib/faker_japanese.rb module inherits from String but within the Faker module. Therefore super in line 27 refers now to Faker::String instead of ruby's String.

Solution

Refer to ruby's String class, e.g. by prefixing the class name with :::

+   class Kanji < ::String
-   class Kanji < String

in following line: