diff --git a/db/migrate/20111018010003_add_back_indexes.rb b/db/migrate/20111018010003_add_back_indexes.rb
index dcd13a6c0..cfc3f9c4c 100644
--- a/db/migrate/20111018010003_add_back_indexes.rb
+++ b/db/migrate/20111018010003_add_back_indexes.rb
@@ -12,7 +12,8 @@ class AddBackIndexes < ActiveRecord::Migration
     remove_index :comments, :column => [:commentable_id, :commentable_type]
     remove_index :photos, :column => :status_message_guid
 
-    change_column :share_visibilities, :shareable_type
-    change_column :comments, :commentable_type
+    change_column :share_visibilities, :shareable_type, :string, :default => "Post", :null => false
+    change_column :comments, :commentable_type, :string, :default => "Post", :null => false
+    # reduce index size
   end
 end