diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 6a35a72..a2a2aac 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -15,6 +15,11 @@ jobs:
       matrix:
         ruby: ["3.2", "3.3", "3.4", "4.0", head, jruby-10.0.0.1]
 
+    env:
+      # Released Bundler is broken on ruby-head due to ruby/ruby#16895.
+      # Use Ruby's bundled Bundler until rubygems/rubygems#9529 is released.
+      BUNDLER_VERSION: ${{ matrix.ruby == 'head' && '0' || '' }}
+
     steps:
       - name: Checkout repository
         uses: actions/checkout@v6
diff --git a/spec/actor_spec.rb b/spec/actor_spec.rb
index c39c310..3aae709 100644
--- a/spec/actor_spec.rb
+++ b/spec/actor_spec.rb
@@ -785,7 +785,7 @@ def call; end
           expect { actor_class.call(per_page: "6") }
             .to raise_error(
               ServiceActor::ArgumentError,
-              /\AThe "per_page" input on ".+" has an error in the code inside "be_in_range": \[ArgumentError\] comparison of String with 3 failed\z/,
+              /\AThe "per_page" input on ".+" has an error in the code inside "be_in_range": \[ArgumentError\] comparison of String with 3 failed(: comparator returned nil)?\z/,
             )
         end
       end