Provide a default value in the case that the ENV variable isn't set

db/migrate/20150808115436_remove_requirement_from_users_invitation_code.rb

@@ -1,5 +1,5 @@
 class RemoveRequirementFromUsersInvitationCode < ActiveRecord::Migration
   def change
-    change_column_null :users, :invitation_code, true, ENV['INVITATION_CODE']
+    change_column_null :users, :invitation_code, true, ENV['INVITATION_CODE'].presence || 'try-huginn'
   end
 end