```ruby
require 'test_helper'
require 'resque/failure/redis'

describe "Resque::Failure::Redis" do
  before do
    @bad_string    = [39, 250, 141, 168, 138, 191, 52, 211, 159, 86, 93, 95, 39].map { |c| c.chr }.join
    exception      = StandardError.exception(@bad_string)
    worker         = Resque::Worker.new(:test)
    queue          = "queue"
    payload        = { "class" => Object, "args" => 3 }
    @redis_backend = Resque::Failure::Redis.new(exception, worker, queue, payload)
  end

  it 'cleans up bad strings before saving the failure, in order to prevent errors on the resque UI' do
    # test assumption: the bad string should not be able to round trip though JSON
    assert_raises(MultiJson::DecodeError, JSON::GeneratorError) {
      MultiJson.decode(MultiJson.encode(@bad_string))
    }

    @redis_backend.save
    Resque::Failure::Redis.all # should not raise an error
  end
end
```

Fix the build on MRI.
user
```ruby
require 'test_helper'
require 'resque/failure/redis'

require 'json'

describe "Resque::Failure::Redis" do
  before do
    @bad_string    = [39, 250, 141, 168, 138, 191, 52, 211, 159, 86, 93, 95, 39].map { |c| c.chr }.join
    exception      = StandardError.exception(@bad_string)
    worker         = Resque::Worker.new(:test)
    queue          = "queue"
    payload        = { "class" => Object, "args" => 3 }
    @redis_backend = Resque::Failure::Redis.new(exception, worker, queue, payload)
  end

  it 'cleans up bad strings before saving the failure, in order to prevent errors on the resque UI' do
    # test assumption: the bad string should not be able to round trip though JSON
    assert_raises(MultiJson::DecodeError, JSON::GeneratorError) {
      MultiJson.decode(MultiJson.encode(@bad_string))
    }

    @redis_backend.save
    Resque::Failure::Redis.all # should not raise an error
  end
end
```
assistant