diff --git a/Gemfile b/Gemfile
index 85120af2d..9d912bee1 100644
--- a/Gemfile
+++ b/Gemfile
@@ -5,6 +5,7 @@ group :development do
   gem "rake-compiler"
   gem "test-unit", "~> 3.0", ">= 3.4.6"
   gem "test-unit-ruby-core"
+  gem "prime"
   # In the case of Ruby whose rdoc is not a default gem.
   gem "rdoc"
 end