diff --git a/puma-acme.gemspec b/puma-acme.gemspec
index d6925b3..690c995 100644
--- a/puma-acme.gemspec
+++ b/puma-acme.gemspec
@@ -22,6 +22,7 @@ Gem::Specification.new do |s|
   s.extra_rdoc_files = ['LICENSE.txt', 'README.md', 'CHANGELOG.md']
 
   s.add_runtime_dependency 'acme-client', '~> 2.0.13'
+  s.add_runtime_dependency 'base64', '~> 0.2'
   s.add_runtime_dependency 'pstore', '~> 0.1'
   s.add_runtime_dependency 'puma', '~> 6.0'
   s.add_runtime_dependency 'sinatra', '>= 3.2'