can't convert false into String (TypeError)

(erb):10:in `concat'
(erb):10:in `eval_rhtml'
/usr/local/httpd/htdocs/mini/diary/tdiary.rb:335:in `eval_rhtml'
(erb):35: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