can't convert nil into Hash (TypeError)

/home/nari/public_html/v2/diary/tdiary.rb:546:in `update'
/home/nari/public_html/v2/diary/tdiary.rb:546:in `transaction'
/usr/local/lib/ruby/1.8/pstore.rb:322:in `transaction'
/usr/local/lib/ruby/1.8/pstore.rb:321:in `catch'
/usr/local/lib/ruby/1.8/pstore.rb:321:in `transaction'
/home/nari/public_html/v2/diary/tdiary.rb:543:in `transaction'
/home/nari/public_html/v2/diary/tdiary.rb:1042:in `initialize'
/home/nari/public_html/v2/diary/index.rb:29:in `new'
/home/nari/public_html/v2/diary/index.rb:29