Redmine

Dears
When I export to PDF or CSV ,it will occur the error as following stack trace.
I try to modify pdf.ruby file,but don't know how to build it.
Is any step to fix this problem for "Big-5" issue?

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database version : MySQL 5.1
Ruby version :ruby 1.8.7
Rails version :Rails 2.3.11
Redmine version :redmine-1.2.1
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error stack trace:
Processing IssuesController#index to pdf (for * at 2012-02-17 20:22:06) [GET]
Parameters: {"format"=>"pdf", "project_id"=>"proj", "action"=>"index", "controller"=>"issues"}

Iconv::InvalidEncoding (invalid encoding ("BIG5", "UTF-8")):
lib/redmine/export/pdf.rb:40:in `initialize'
lib/redmine/export/pdf.rb:40:in `new'
lib/redmine/export/pdf.rb:40:in `initialize'
lib/redmine/export/pdf.rb:146:in `new'
lib/redmine/export/pdf.rb:146:in `issues_to_pdf'
app/controllers/issues_controller.rb:96:in `index'
app/controllers/issues_controller.rb:91:in `index'
/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 /opt/redmine/public/500.html (500 Internal Server Error)