can't convert nil into Hash (TypeError)

/virtual/keijis/public_html/tdiary/tdiary.rb:543:in `update'
/virtual/keijis/public_html/tdiary/tdiary.rb:543: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/keijis/public_html/tdiary/tdiary.rb:540:in `transaction'
/virtual/keijis/public_html/tdiary/tdiary.rb:1038:in `initialize'
/virtual/keijis/public_html/tdiary/index.rb:28:in `new'
/virtual/keijis/public_html/tdiary/index.rb:28