diff --git a/example/zoo-app/Gemfile b/example/zoo-app/Gemfile
index 61b86165..63788171 100644
--- a/example/zoo-app/Gemfile
+++ b/example/zoo-app/Gemfile
@@ -8,5 +8,5 @@ group :development, :test do
 end
 
 gem 'rake'
-gem 'rack', '~>2.2'
+gem 'rack', '~> 2.2', '>= 2.2.6.4'
 gem 'httparty'