diff --git a/mailcatcher.gemspec b/mailcatcher.gemspec
index 696231b..e29d19c 100644
--- a/mailcatcher.gemspec
+++ b/mailcatcher.gemspec
@@ -33,6 +33,7 @@ Gem::Specification.new do |s|
   s.add_dependency "activesupport", "~> 4.0"
   s.add_dependency "eventmachine", "1.0.9.1"
   s.add_dependency "mail", "~> 2.3"
+  s.add_dependency "rack", "~> 1.5"
   s.add_dependency "sinatra", "~> 1.2"
   s.add_dependency "sqlite3", "~> 1.3"
   s.add_dependency "thin", "~> 1.5.0"