Redmine

When exporting a wiki page to PDF I receive this error (from production.log):

NoMethodError (undefined method `downcase' for nil:NilClass):
  lib/redmine/export/pdf.rb:120:in `RDMwriteHTMLCell'
  lib/redmine/export/pdf.rb:509:in `wiki_to_pdf'
  app/controllers/wiki_controller.rb:76:in `show'
  passenger (3.0.6) lib/phusion_passenger/rack/request_handler.rb:96:in `process_request'
  passenger (3.0.6) lib/phusion_passenger/abstract_request_handler.rb:513:in `accept_and_process_next_request'
  passenger (3.0.6) lib/phusion_passenger/abstract_request_handler.rb:274:in `main_loop'
  passenger (3.0.6) lib/phusion_passenger/classic_rails/application_spawner.rb:321:in `start_request_handler'
  passenger (3.0.6) lib/phusion_passenger/classic_rails/application_spawner.rb:275:in `send'
  passenger (3.0.6) lib/phusion_passenger/classic_rails/application_spawner.rb:275:in `handle_spawn_application'
  passenger (3.0.6) lib/phusion_passenger/utils.rb:479:in `safe_fork'
  passenger (3.0.6) lib/phusion_passenger/classic_rails/application_spawner.rb:270:in `handle_spawn_application'
  passenger (3.0.6) lib/phusion_passenger/abstract_server.rb:357:in `__send__'
  passenger (3.0.6) lib/phusion_passenger/abstract_server.rb:357:in `server_main_loop'
  passenger (3.0.6) lib/phusion_passenger/abstract_server.rb:206:in `start_synchronously'
  passenger (3.0.6) lib/phusion_passenger/abstract_server.rb:180:in `start'
  passenger (3.0.6) lib/phusion_passenger/classic_rails/application_spawner.rb:149:in `start'
  passenger (3.0.6) lib/phusion_passenger/spawn_manager.rb:219:in `spawn_rails_application'
  passenger (3.0.6) lib/phusion_passenger/abstract_server_collection.rb:132:in `lookup_or_add'
  passenger (3.0.6) lib/phusion_passenger/spawn_manager.rb:214:in `spawn_rails_application'
  passenger (3.0.6) lib/phusion_passenger/abstract_server_collection.rb:82:in `synchronize'
  passenger (3.0.6) lib/phusion_passenger/abstract_server_collection.rb:79:in `synchronize'
  passenger (3.0.6) lib/phusion_passenger/spawn_manager.rb:213:in `spawn_rails_application'
  passenger (3.0.6) lib/phusion_passenger/spawn_manager.rb:132:in `spawn_application'
  passenger (3.0.6) lib/phusion_passenger/spawn_manager.rb:275:in `handle_spawn_application'
  passenger (3.0.6) lib/phusion_passenger/abstract_server.rb:357:in `__send__'
  passenger (3.0.6) lib/phusion_passenger/abstract_server.rb:357:in `server_main_loop'
  passenger (3.0.6) lib/phusion_passenger/abstract_server.rb:206:in `start_synchronously'
  passenger (3.0.6) helper-scripts/passenger-spawn-server:99

Rendering /var/www/redmine-1.3.1/public/500.html (500 Internal Server Error)

I found a month-old report in the forum (http://www.redmine.org/boards/2/topics/29010) without any response.

I'm using Redmine 1.3.1.stable (MySQL)