diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 6e359eca8..bb4eadbdd 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -33,6 +33,7 @@ jobs:
           - "3.2"
           - "3.3"
           - "3.4"
+          - "4.0"
           - ruby-head
           - jruby-9.4
         task:
diff --git a/Gemfile b/Gemfile
index 7613f51e7..26a888bb5 100644
--- a/Gemfile
+++ b/Gemfile
@@ -5,6 +5,7 @@ source 'https://rubygems.org'
 gemspec
 
 gem 'bump'
+gem 'irb' # undeclared dependency of yard
 gem 'rack'
 gem 'rake'
 gem 'rspec', '~> 3.11'