now reverting migration will raise AR error

db/migrate/20150929142405_change_postalcode_format_in_venues.rb

@@ -1,5 +1,9 @@
 class ChangePostalcodeFormatInVenues < ActiveRecord::Migration
-  def change
+  def up
     change_column :venues, :postalcode, :string
   end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration.new('Cannot reverse migration.')
+  end
 end