diff --git a/test/setup_bundler_test.rb b/test/setup_bundler_test.rb
index f842650129..562a27812a 100644
--- a/test/setup_bundler_test.rb
+++ b/test/setup_bundler_test.rb
@@ -546,7 +546,7 @@ def test_ruby_lsp_rails_detection_handles_lang_from_environment
       end
     end
   ensure
-    Encoding.default_external = original_encoding
+    Encoding.default_external = T.must(original_encoding)
   end
 
   def test_recovers_from_stale_lockfiles