can't convert nil into String (TypeError)

(eval):5:in `gsub'
(eval):5:in `strftime'
(erb):3: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