diff --git a/httpi.gemspec b/httpi.gemspec
index 74ade80..bba9a55 100644
--- a/httpi.gemspec
+++ b/httpi.gemspec
@@ -22,6 +22,7 @@ Gem::Specification.new do |s|
   s.license = 'MIT'
 
   s.add_dependency 'rack', '< 3'
+  s.add_dependency 'nkf'
 
   s.add_development_dependency 'rubyntlm', '~> 0.3.2'
   s.add_development_dependency 'rake',     '~> 13.0'