diff --git a/ext/nkf/nkf.gemspec b/ext/nkf/nkf.gemspec
index 92ab0f22fe..097a9485ed 100644
--- a/ext/nkf/nkf.gemspec
+++ b/ext/nkf/nkf.gemspec
@@ -31,4 +31,5 @@ Gem::Specification.new do |spec|
   spec.bindir        = "exe"
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
+  spec.extensions    = ["ext/nkf/extconf.rb"]
 end