diff --git a/Gemfile b/Gemfile
index f533d5b..75af539 100644
--- a/Gemfile
+++ b/Gemfile
@@ -74,6 +74,8 @@ end
 
 group :test do
   gem "shoulda", "~> 2.11"
+  # Shoulda does not work nice on Ruby 1.9.3 and seems to need test-unit explicitely.
+  gem "test-unit", :platforms => [:mri_19]
   gem "mocha"
 end