undefined method `add_comment' for nil:NilClass (NoMethodError)

/home/mikan/public_html/tdiary/tdiary/defaultio.rb:43:in `restore_comment'
/home/mikan/public_html/tdiary/tdiary/defaultio.rb:34:in `each'
/home/mikan/public_html/tdiary/tdiary/defaultio.rb:34:in `restore_comment'
/home/mikan/public_html/tdiary/tdiary/defaultio.rb:33:in `open'
/home/mikan/public_html/tdiary/tdiary/defaultio.rb:33:in `restore_comment'
/home/mikan/public_html/tdiary/tdiary/defaultio.rb:137:in `transaction'
/home/mikan/public_html/tdiary/tdiary.rb:1661:in `initialize'
/home/mikan/public_html/tdiary/index.rb:44:in `new'
/home/mikan/public_html/tdiary/index.rb:44