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

/usr/local/httpd/htdocs/mini/diary/tdiary/defaultio.rb:166:in `calendar'
/usr/local/httpd/htdocs/mini/diary/tdiary/defaultio.rb:162:in `calendar'
/usr/local/httpd/htdocs/mini/diary/tdiary/defaultio.rb:160:in `calendar'
/usr/local/httpd/htdocs/mini/diary/tdiary.rb:1024:in `calendar'
/usr/local/httpd/htdocs/mini/diary/tdiary.rb:914:in `load_plugins'
/usr/local/httpd/htdocs/mini/diary/tdiary.rb:882: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