Redmine

I have a parent item and few child items.
When I edit a child item and enter a date (eiter starting or closing date) and try to save my changes, I get an error.
The item I am saving has no child items.

Log details:
Processing IssuesController#update (for xxx.xxx.xxx.xxx at 2011-05-13 11:10:15) [PUT]
Parameters: {"time_entry"=>{"comments"=>"", "activity_id"=>"9", "hours"=>""}, "commit"=>"Wyślij", "notes"=>"", "action"=>"update", "_method"=>"put", "authenticity_token"=>"xxxxxxxxxxxxxxxxxxx", "id"=>"12", "issue"=>{"start_date"=>"2011-05-13", "custom_field_values"=>{"2"=>"Minor change", "4"=>""}, "estimated_hours"=>"", "priority_id"=>"4", "parent_issue_id"=>"9", "lock_version"=>"2", "fixed_version_id"=>"3", "done_ratio"=>"0", "assigned_to_id"=>"", "subject"=>"Business concept", "category_id"=>"2", "tracker_id"=>"2", "due_date"=>"2011-05-20", "status_id"=>"1", "description"=>""}, "controller"=>"issues", "attachments"=>{"1"=>{"description"=>""}}}

NoMethodError (undefined method `<=>' for nil:NilClass):
app/models/issue.rb:553:in `save_issue_with_child_records'
app/models/issue.rb:535:in `save_issue_with_child_records'
app/controllers/issues_controller.rb:170:in `update'
passenger (3.0.7) lib/phusion_passenger/rack/request_handler.rb:96:in `process_request'
passenger (3.0.7) lib/phusion_passenger/abstract_request_handler.rb:513:in `accept_and_process_next_request'
passenger (3.0.7) lib/phusion_passenger/abstract_request_handler.rb:274:in `main_loop'
passenger (3.0.7) lib/phusion_passenger/classic_rails/application_spawner.rb:321:in `start_request_handler'
passenger (3.0.7) lib/phusion_passenger/classic_rails/application_spawner.rb:275:in `send'
passenger (3.0.7) lib/phusion_passenger/classic_rails/application_spawner.rb:275:in `handle_spawn_application'
passenger (3.0.7) lib/phusion_passenger/utils.rb:479:in `safe_fork'
passenger (3.0.7) lib/phusion_passenger/classic_rails/application_spawner.rb:270:in `handle_spawn_application'
passenger (3.0.7) lib/phusion_passenger/abstract_server.rb:357:in `__send__'
passenger (3.0.7) lib/phusion_passenger/abstract_server.rb:357:in `server_main_loop'
passenger (3.0.7) lib/phusion_passenger/abstract_server.rb:206:in `start_synchronously'
passenger (3.0.7) lib/phusion_passenger/abstract_server.rb:180:in `start'
passenger (3.0.7) lib/phusion_passenger/classic_rails/application_spawner.rb:149:in `start'
passenger (3.0.7) lib/phusion_passenger/spawn_manager.rb:219:in `spawn_rails_application'
passenger (3.0.7) lib/phusion_passenger/abstract_server_collection.rb:132:in `lookup_or_add'
passenger (3.0.7) lib/phusion_passenger/spawn_manager.rb:214:in `spawn_rails_application'
passenger (3.0.7) lib/phusion_passenger/abstract_server_collection.rb:82:in `synchronize'
passenger (3.0.7) lib/phusion_passenger/abstract_server_collection.rb:79:in `synchronize'
passenger (3.0.7) lib/phusion_passenger/spawn_manager.rb:213:in `spawn_rails_application'
passenger (3.0.7) lib/phusion_passenger/spawn_manager.rb:132:in `spawn_application'
passenger (3.0.7) lib/phusion_passenger/spawn_manager.rb:275:in `handle_spawn_application'
passenger (3.0.7) lib/phusion_passenger/abstract_server.rb:357:in `__send__'
passenger (3.0.7) lib/phusion_passenger/abstract_server.rb:357:in `server_main_loop'
passenger (3.0.7) lib/phusion_passenger/abstract_server.rb:206:in `start_synchronously'
passenger (3.0.7) helper-scripts/passenger-spawn-server:99