diff --git a/prometheus-client.gemspec b/prometheus-client.gemspec
index 029ff4f1..a421f0ac 100644
--- a/prometheus-client.gemspec
+++ b/prometheus-client.gemspec
@@ -15,6 +15,8 @@ Gem::Specification.new do |s|
   s.files             = %w(README.md LICENSE) + Dir.glob('{lib/**/*}')
   s.require_paths     = ['lib']
 
+  s.add_dependency 'base64'
+
   s.add_development_dependency 'benchmark-ips'
   s.add_development_dependency 'concurrent-ruby'
   s.add_development_dependency 'timecop'