diff --git a/app/models/post_archive.rb b/app/models/post_archive.rb
index fa4e044c2..8b92f1560 100644
--- a/app/models/post_archive.rb
+++ b/app/models/post_archive.rb
@@ -221,6 +221,8 @@ class PostArchive < ActiveRecord::Base
 
     if previous
       delta[:unchanged_tags] = tag_array & previous.tag_array
+    else
+      delta[:unchanged_tags] = []
     end
 
     delta