diff --git a/lib/loofah.rb b/lib/loofah.rb
index d4a1be2..5dbd2ec 100644
--- a/lib/loofah.rb
+++ b/lib/loofah.rb
@@ -36,7 +36,7 @@ def html5_support?
 require_relative "loofah/html4/document"
 require_relative "loofah/html4/document_fragment"
 
-if Nokogiri.respond_to?(:uses_gumbo?) && Nokogiri.uses_gumbo?
+if Loofah.html5_support?
   require_relative "loofah/html5/document"
   require_relative "loofah/html5/document_fragment"
 end
@@ -123,19 +123,19 @@ def scrub_html5_fragment(string_or_io, method)
       end
     else
       def html5_document(*args, &block)
-        raise NotImplementedError, "HTML5 is not supported by your version of Nokogiri"
+        raise NotImplementedError, "Loofah::HTML5 is not supported by your version of Nokogiri"
       end
 
       def html5_fragment(*args, &block)
-        raise NotImplementedError, "HTML5 is not supported by your version of Nokogiri"
+        raise NotImplementedError, "Loofah::HTML5 is not supported by your version of Nokogiri"
       end
 
       def scrub_html5_document(string_or_io, method)
-        raise NotImplementedError, "HTML5 is not supported by your version of Nokogiri"
+        raise NotImplementedError, "Loofah::HTML5 is not supported by your version of Nokogiri"
       end
 
       def scrub_html5_fragment(string_or_io, method)
-        raise NotImplementedError, "HTML5 is not supported by your version of Nokogiri"
+        raise NotImplementedError, "Loofah::HTML5 is not supported by your version of Nokogiri"
       end
     end
 
diff --git a/test/unit/test_api.rb b/test/unit/test_api.rb
index 30cc288..be0b6bf 100644
--- a/test/unit/test_api.rb
+++ b/test/unit/test_api.rb
@@ -109,6 +109,11 @@ class UnitTestApi < Loofah::TestCase
           assert_raises(NotImplementedError) { Loofah.html5_fragment(html) }
           assert_raises(NotImplementedError) { Loofah.scrub_html5_fragment(html, :strip) }
         end
+
+        it "does not implement classes" do
+          assert_raises(NameError) { Loofah::HTML5::Document }
+          assert_raises(NameError) { Loofah::HTML5::DocumentFragment }
+        end
       end
     end