diff --git a/ruby-saml.gemspec b/ruby-saml.gemspec
index 5383b56d..50ceef0f 100644
--- a/ruby-saml.gemspec
+++ b/ruby-saml.gemspec
@@ -25,4 +25,5 @@ Gem::Specification.new do |s|
 
   s.add_dependency('nokogiri', '>= 1.13.10')
   s.add_dependency('rexml')
+  s.add_dependency('base64')
 end