TypeError (can't convert nil into String) while Processing IssuesController#show to pdf (Bug #1089)

I am getting the same error as here: #561 although I am running version v3.1.0. I have the changes made by the patch in that issue already.

I have enabled backlogs version 0.9.26, but I don't know if that is causing the problem. I'm guessing it probably is.

Here is the output from my log:

Processing IssuesController#show to pdf (for 75.189.249.9 at 2012-07-22 18:36:23) [GET]
  Parameters: {"id"=>"17", "format"=>"pdf", "controller"=>"issues", "action"=>"show"}

TypeError (can't convert nil into String):
  lib/redmine/export/pdf.rb:417:in `+'
  lib/redmine/export/pdf.rb:417:in `issue_to_pdf'
  lib/redmine/export/pdf.rb:416:in `each'
  lib/redmine/export/pdf.rb:416:in `issue_to_pdf'
  lib/redmine/export/pdf.rb:410:in `each'
  lib/redmine/export/pdf.rb:410:in `issue_to_pdf'
  app/controllers/issues_controller.rb:106
  app/controllers/issues_controller.rb:102:in `show'
  passenger (3.0.1) lib/phusion_passenger/rack/request_handler.rb:96:in `process_request'
  passenger (3.0.1) lib/phusion_passenger/abstract_request_handler.rb:513:in `accept_and_process_next_request'
  passenger (3.0.1) lib/phusion_passenger/abstract_request_handler.rb:274:in `main_loop'
  passenger (3.0.1) lib/phusion_passenger/classic_rails/application_spawner.rb:321:in `start_request_handler'
  passenger (3.0.1) lib/phusion_passenger/classic_rails/application_spawner.rb:275:in `send'
  passenger (3.0.1) lib/phusion_passenger/classic_rails/application_spawner.rb:275:in `handle_spawn_application'
  passenger (3.0.1) lib/phusion_passenger/utils.rb:479:in `safe_fork'
  passenger (3.0.1) lib/phusion_passenger/classic_rails/application_spawner.rb:270:in `handle_spawn_application'
  passenger (3.0.1) lib/phusion_passenger/abstract_server.rb:357:in `__send__'
  passenger (3.0.1) lib/phusion_passenger/abstract_server.rb:357:in `server_main_loop'
  passenger (3.0.1) lib/phusion_passenger/abstract_server.rb:206:in `start_synchronously'
  passenger (3.0.1) lib/phusion_passenger/abstract_server.rb:180:in `start'
  passenger (3.0.1) lib/phusion_passenger/classic_rails/application_spawner.rb:149:in `start'
  passenger (3.0.1) lib/phusion_passenger/spawn_manager.rb:219:in `spawn_rails_application'
  passenger (3.0.1) lib/phusion_passenger/abstract_server_collection.rb:132:in `lookup_or_add'
  passenger (3.0.1) lib/phusion_passenger/spawn_manager.rb:214:in `spawn_rails_application'
  passenger (3.0.1) lib/phusion_passenger/abstract_server_collection.rb:82:in `synchronize'
  passenger (3.0.1) lib/phusion_passenger/abstract_server_collection.rb:79:in `synchronize'
  passenger (3.0.1) lib/phusion_passenger/spawn_manager.rb:213:in `spawn_rails_application'
  passenger (3.0.1) lib/phusion_passenger/spawn_manager.rb:132:in `spawn_application'
  passenger (3.0.1) lib/phusion_passenger/spawn_manager.rb:275:in `handle_spawn_application'
  passenger (3.0.1) lib/phusion_passenger/abstract_server.rb:357:in `__send__'
  passenger (3.0.1) lib/phusion_passenger/abstract_server.rb:357:in `server_main_loop'
  passenger (3.0.1) lib/phusion_passenger/abstract_server.rb:206:in `start_synchronously'
  passenger (3.0.1) helper-scripts/passenger-spawn-server:99

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