diff --git a/ext/stringio/stringio.gemspec b/ext/stringio/stringio.gemspec
index c765242f35..d12c648a7d 100644
--- a/ext/stringio/stringio.gemspec
+++ b/ext/stringio/stringio.gemspec
@@ -30,4 +30,5 @@ Gem::Specification.new do |s|
   # s.signing_key = File.expand_path("~/.ssh/gem-private_key.pem") if $0 =~ /gem\z/
 
   s.add_development_dependency 'rake-compiler'
+  s.add_development_dependency 'test-unit'
 end