diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 479a27169b..37f8b4889c 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -12,6 +12,7 @@ jobs:
   test:
     needs: ruby-versions
     strategy:
+      fail-fast: false
       matrix:
         ruby: ${{ fromJson(needs.ruby-versions.outputs.versions) }}
         os: [ubuntu-latest, macos-latest, windows-latest]
diff --git a/lib/rdoc/parser.rb b/lib/rdoc/parser.rb
index 425105effa..21ba59e33c 100644
--- a/lib/rdoc/parser.rb
+++ b/lib/rdoc/parser.rb
@@ -128,6 +128,7 @@ def self.can_parse_by_name file_name
       case mode = check_modeline(file_name)
       when nil, 'rdoc' then # continue
       else
+        raise "#{mode.inspect} should be a String" unless String === mode
         RDoc::Parser.parsers.find { |_, p| return p if mode.casecmp?(p.name[/\w+\z/]) }
         return nil
       end