diff --git a/db/migrate/20151001024122_mapping_polymorphism.rb b/db/migrate/20151001024122_mapping_polymorphism.rb
index 6ba88f7c..e41233f6 100644
--- a/db/migrate/20151001024122_mapping_polymorphism.rb
+++ b/db/migrate/20151001024122_mapping_polymorphism.rb
@@ -17,6 +17,7 @@ class MappingPolymorphism < ActiveRecord::Migration
       unless mapping.synapse_id.nil?
         mapping.mappable = Synapse.find(mapping.synapse_id)
       else
+        next if mapping.topic_id == 0
         mapping.mappable = Topic.find(mapping.topic_id)
       end
       mapping.save