diff --git a/app/models/concerns/tags_substitution_concern.rb b/app/models/concerns/tags_substitution_concern.rb
index 8512814db4c..3795fcec106 100644
--- a/app/models/concerns/tags_substitution_concern.rb
+++ b/app/models/concerns/tags_substitution_concern.rb
@@ -12,7 +12,7 @@ module TagsParser
     extend self
 
     def parse(io)
-      doc.parse io
+      doc.parse +io # parsby mutates the StringIO during parsing!
     end
 
     def self.normalize(str)