diff --git a/elasticsearch-dsl.gemspec b/elasticsearch-dsl.gemspec
index ae5c20c..f62d973 100644
--- a/elasticsearch-dsl.gemspec
+++ b/elasticsearch-dsl.gemspec
@@ -37,4 +37,8 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'yard'
   s.add_development_dependency 'cane'
   s.add_development_dependency 'pry'
+
+  if defined?(RUBY_VERSION) && RUBY_VERSION > '2.2'
+    s.add_development_dependency "test-unit", '~> 2'
+  end
 end