diff --git a/mutex_m.gemspec b/mutex_m.gemspec
index f99eb45..eb36419 100644
--- a/mutex_m.gemspec
+++ b/mutex_m.gemspec
@@ -18,6 +18,6 @@ Gem::Specification.new do |spec|
   spec.metadata["bug_tracker_uri"] = "#{spec.homepage}/issues"
   spec.licenses = %w[Ruby BSD-2-Clause]
 
-  spec.files = %w[LICENSE.txt README.md lib/mutex_m.rb]
+  spec.files = %w[LICENSE.txt README.md lib/mutex_m.rb sig/mutex_m.rbs]
   spec.required_ruby_version = ">= 2.5"
 end