sitemap generator does not work properly / Rails Application

Question

i built a simple sitemap with the gem sitemap generator. It is supposed to list the job pages of my application. E.g. www.mydomain.com/vacancies/24

There is a bug that I cannot fix myself for days:

It shows the wrong indexes (that do not exist). E.g. www.mydomain.com/vacancies/645 enter code hereHere is my sitemap.rb file:

require 'rubygems'
require 'sitemap_generator'

SitemapGenerator::Sitemap.default_host = "http://www.frankfurtstartupjobs.com"

SitemapGenerator::Sitemap.create do

  Vacancy.find_each do |vacancy|
    add vacancy_path(vacancy), :lastmod => vacancy.updated_at, :priority => 0.5
  end
end