can't convert nil into Hash (TypeError)

/virtual/epi/public_html/diary/tdiary.rb:546:in `update'
/virtual/epi/public_html/diary/tdiary.rb:546:in `transaction'
/usr/local/lib/ruby/1.8/pstore.rb:319:in `transaction'
/usr/local/lib/ruby/1.8/pstore.rb:318:in `catch'
/usr/local/lib/ruby/1.8/pstore.rb:318:in `transaction'
/virtual/epi/public_html/diary/tdiary.rb:543:in `transaction'
/virtual/epi/public_html/diary/tdiary.rb:902:in `load'
/virtual/epi/public_html/diary/tdiary.rb:892:in `initialize'
/virtual/epi/public_html/diary/index.rb:24:in `new'
/virtual/epi/public_html/diary/index.rb:24