diff --git a/.github/CONTRIBUTING.md b/.github/CONTRIBUTING.md
index 40634fa6..aecfe29e 100644
--- a/.github/CONTRIBUTING.md
+++ b/.github/CONTRIBUTING.md
@@ -18,7 +18,9 @@ Please use our [bug report template][1].
 
 ## Development
 
-Install gems with `bundle exec appraisal install`.
+Install gems with `bundle exec appraisal install`. This requires ruby >= 2.0.
+(It is still possible to run the `ar-4.2` gemfile locally on ruby 1.9.3, but
+not the `ar-5.0` gemfile.)
 
 Testing is a little awkward because the test suite:
 
diff --git a/paper_trail.gemspec b/paper_trail.gemspec
index 2792cb10..1b7cf97d 100644
--- a/paper_trail.gemspec
+++ b/paper_trail.gemspec
@@ -44,6 +44,10 @@ has been destroyed.
   s.add_development_dependency "rubocop", "~> 0.41.1"
   s.add_development_dependency "timecop", "~> 0.8.0"
 
+  if ::Gem.ruby_version < ::Gem::Version.new("2.0.0")
+    s.add_development_dependency "rack", "< 2"
+  end
+
   if defined?(JRUBY_VERSION)
     s.add_development_dependency "activerecord-jdbcsqlite3-adapter", "~> 1.3.15"
     s.add_development_dependency "activerecord-jdbcpostgresql-adapter", "~> 1.3.15"