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

/virtual/neguse/public_html/blog/tdiary/defaultio.rb:40:in `restore_comment'
/virtual/neguse/public_html/blog/tdiary/defaultio.rb:30:in `open'
/virtual/neguse/public_html/blog/tdiary/defaultio.rb:30:in `restore_comment'
/virtual/neguse/public_html/blog/tdiary/defaultio.rb:134:in `transaction'
/virtual/neguse/public_html/blog/tdiary.rb:1558:in `initialize'
/virtual/neguse/public_html/blog/index.rb:44:in `new'
/virtual/neguse/public_html/blog/index.rb:44