Ruby2Ruby destroys its input

Gemfile

source 'http://rubygems.org'

gem 'ruby2ruby'
gem 'rspec'

output.txt

~/ca7cdca66e4545c952a0(master)$ be rspec ruby2ruby_should_not_destroy_its_input_spec.rb 
F

Failures:

  1) Ruby2Ruby is non-destructive
     Failure/Error: expect(sexp).to eq(s(:call, s(:call, nil, :a), :b))
       
       expected: s(:call, s(:call, nil, :a), :b)
            got: s()
       
       (compared using ==)
       
       Diff:
       @@ -1,2 +1,2 @@
       -[:call, [:call, nil, :a], :b]
       +[]
     # ./ruby2ruby_should_not_destroy_its_input_spec.rb:8:in `block (2 levels) in <top (required)>'

Finished in 0.00212 seconds (files took 0.0897 seconds to load)
1 example, 1 failure

Failed examples:

rspec ./ruby2ruby_should_not_destroy_its_input_spec.rb:4 # Ruby2Ruby is non-destructive

ruby2ruby_should_not_destroy_its_input_spec.rb

require 'ruby2ruby'

describe "Ruby2Ruby" do
  it "is non-destructive" do
    sexp = s(:call, s(:call, nil, :a), :b)
    Ruby2Ruby.new.process(sexp)

    expect(sexp).to eq(s(:call, s(:call, nil, :a), :b))
  end
end