diff --git a/Gemfile b/Gemfile
index 649e912e..30e18ca8 100644
--- a/Gemfile
+++ b/Gemfile
@@ -17,6 +17,7 @@ platforms :jruby do
 end
 
 platforms :rbx do
+  gem 'json'
   gem 'rubysl', '~> 2.0'
   gem 'racc', '~> 1.4.10'
 end