Redmine

Hi,my dear friends,
I got a internal error,when I try to add a task to a user sotry in scrum_pm,
what's the problum,how to resolve it?thans alot

in production.log，logs these:

Processing IssuesController#create (for 192.168.0.4 at 2010-12-27 15:42:41) [POST]
Parameters: {"commit"=>"创建", "project_id"=>"test02", "action"=>"create", "authenticity_token"=>"5qaN+5IlWl7OnDgD6v6ORUDTzpf8HxhyZMY
LhamIqBU=", "issue"=>{"start_date"=>"2010-12-27", "user_story_id"=>"1", "estimated_hours"=>"", "priority_id"=>"4", "parent_issue_id"=>"
", "fixed_version_id"=>"1", "done_ratio"=>"0", "assigned_to_id"=>"", "subject"=>"fffffffffffff", "tracker_id"=>"1", "due_date"=>"", "st
atus_id"=>"1", "description"=>"adffffffffffffff"}, "controller"=>"issues", "attachments"=>{"1"=>{"description"=>""}}}

TypeError (can't convert nil into String):
lib/redmine/hook.rb:63:in `send'
lib/redmine/hook.rb:63:in `call_hook'
lib/redmine/hook.rb:63:in `each'
lib/redmine/hook.rb:63:in `call_hook'
lib/redmine/hook.rb:60:in `call_hook'
lib/redmine/hook.rb:141:in `call_hook'
app/controllers/issues_controller.rb:128:in `create'
/usr/local/lib/ruby/1.8/webrick/httpserver.rb:104:in `service'
/usr/local/lib/ruby/1.8/webrick/httpserver.rb:65:in `run'
/usr/local/lib/ruby/1.8/webrick/server.rb:173:in `start_thread'
/usr/local/lib/ruby/1.8/webrick/server.rb:162:in `start'
/usr/local/lib/ruby/1.8/webrick/server.rb:162:in `start_thread'
/usr/local/lib/ruby/1.8/webrick/server.rb:95:in `start'
/usr/local/lib/ruby/1.8/webrick/server.rb:92:in `each'
/usr/local/lib/ruby/1.8/webrick/server.rb:92:in `start'
/usr/local/lib/ruby/1.8/webrick/server.rb:23:in `start'
/usr/local/lib/ruby/1.8/webrick/server.rb:82:in `start'

Rendering /usr/local/redmine/public/500.html (500 Internal Server Error)