diff --git a/idempotency_key.gemspec b/idempotency_key.gemspec
index 3c4c17b..ed6bbb0 100644
--- a/idempotency_key.gemspec
+++ b/idempotency_key.gemspec
@@ -24,4 +24,6 @@ Gem::Specification.new do |spec|
   spec.bindir = "exe"
   spec.executables = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
+
+  spec.add_dependency "rack", ">= 2.0.1", "< 4"
 end