undefined method `+' for false:FalseClass (NoMethodError)

/usr/local/httpd/htdocs/mini/diary/tdiary/tdiary_style.rb:62:in `categorized_subtitle'
/usr/local/httpd/htdocs/mini/diary/tdiary/tdiary_style.rb:153:in `to_html4'
/usr/local/httpd/htdocs/mini/diary/tdiary/tdiary_style.rb:118:in `each_section'
/usr/local/httpd/htdocs/mini/diary/tdiary/tdiary_style.rb:117:in `each_section'
/usr/local/httpd/htdocs/mini/diary/tdiary/tdiary_style.rb:142:in `to_html4'
/usr/local/httpd/htdocs/mini/diary/tdiary/tdiary_style.rb:135:in `to_html'
(erb):11:in `eval_rhtml'
/usr/local/httpd/htdocs/mini/diary/tdiary.rb:335:in `eval_rhtml'
(erb):31:in `do_eval_rhtml'
/usr/local/httpd/htdocs/mini/diary/tdiary.rb:1457:in `each_day'
/usr/local/httpd/htdocs/mini/diary/tdiary.rb:1454:in `each_day'
(erb):30:in `do_eval_rhtml'
/usr/local/httpd/htdocs/mini/diary/tdiary.rb:898:in `do_eval_rhtml'
/usr/local/httpd/htdocs/mini/diary/tdiary.rb:854:in `eval_rhtml'
/usr/local/httpd/htdocs/mini/diary/index.rb:71