diff --git a/.github/workflows/rubocop.yml b/.github/workflows/rubocop.yml
index 40903c2..43d7818 100644
--- a/.github/workflows/rubocop.yml
+++ b/.github/workflows/rubocop.yml
@@ -10,6 +10,8 @@ jobs:
   build:
 
     runs-on: ubuntu-latest
+    env:
+      BUNDLER_VERSION: 4.0.6
 
     steps:
     - uses: actions/checkout@v6
@@ -17,6 +19,12 @@ jobs:
       uses: ruby/setup-ruby@v1
       with:
         ruby-version: "4.0"
+    - name: Pin Bundler to 4.0.6 (workaround for ruby/rubygems#9536)
+      run: |
+        gem install bundler -v 4.0.6
+        ruby --version
+        gem --version
+        bundle --version
     - name: Build and run RuboCop
       run: |
         BUNDLE_ONLY=rubocop bundle install --jobs 4 --retry 3