500 Internal Server Error

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

(plugin/title_tag.rb):31:in `all_subtitles_to_html'
/home/harunaru/tdiary/tdiary/wiki_style.rb:248:in `each_section'
/home/harunaru/tdiary/tdiary/wiki_style.rb:247:in `each'
/home/harunaru/tdiary/tdiary/wiki_style.rb:247:in `each_section'
(plugin/title_tag.rb):30:in `all_subtitles_to_html'
(plugin/title_tag.rb):58:in `title_tag'
(plugin/00default.rb):221:in `load_plugin'
/home/harunaru/tdiary/tdiary/plugin.rb:108:in `call'
/home/harunaru/tdiary/tdiary/plugin.rb:108:in `header_proc'
/home/harunaru/tdiary/tdiary/plugin.rb:107:in `each'
/home/harunaru/tdiary/tdiary/plugin.rb:107:in `header_proc'
(TDiary::Plugin#eval_src):4:in `eval_src'
/home/harunaru/tdiary/tdiary/plugin.rb:96:in `eval_src'
/home/harunaru/tdiary/tdiary/core_ext.rb:82:in `safe'
/home/harunaru/tdiary/tdiary/core_ext.rb:80:in `call'
/home/harunaru/tdiary/tdiary/core_ext.rb:80:in `safe'
/home/harunaru/tdiary/tdiary/plugin.rb:95:in `eval_src'
/home/harunaru/tdiary/tdiary.rb:214:in `do_eval_rhtml'
/home/harunaru/tdiary/tdiary.rb:162:in `eval_rhtml'
/home/harunaru/tdiary/tdiary.rb:779:in `eval_rhtml'
/home/harunaru/tdiary/tdiary/dispatcher/index_main.rb:43:in `run'
/home/harunaru/tdiary/tdiary/dispatcher/index_main.rb:6:in `run'
/home/harunaru/tdiary/tdiary/dispatcher.rb:21:in `dispatch_cgi'
/home/harunaru/tdiary/index.rb:36
index.rb:2:in `require'
index.rb:2