diff --git a/lib/topic_assigner.rb b/lib/topic_assigner.rb
index fad8d2c..a11ae5b 100644
--- a/lib/topic_assigner.rb
+++ b/lib/topic_assigner.rb
@@ -60,7 +60,7 @@ class ::TopicAssigner
       return unless force || post.topic.custom_fields[ASSIGNED_TO_ID].nil?
 
       # remove quotes, oneboxes and code blocks
-      doc = Nokogiri::HTML.fragment(post.cooked)
+      doc = Nokogiri::HTML5.fragment(post.cooked)
       doc.css(".quote, .onebox, pre, code").remove
       text = doc.text.strip