diff --git a/rack-session.gemspec b/rack-session.gemspec
index aed2fde..1acc6ef 100644
--- a/rack-session.gemspec
+++ b/rack-session.gemspec
@@ -16,6 +16,7 @@ Gem::Specification.new do |spec|
 
   spec.required_ruby_version = ">= 2.4.0"
 
+  spec.add_dependency "base64"
   spec.add_dependency "rack", ">= 3.0.0"
 
   spec.add_development_dependency "bundler"