500 Internal Server Error

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

(plugin/archive.rb):60:in `navi_user_blog_category'
./tdiary.rb:105:in `join'
./tdiary.rb:105:in `safe'
./tdiary.rb:749:in `eval_src'
./tdiary.rb:1095:in `do_eval_rhtml'
./tdiary.rb:1037:in `eval_rhtml'
index.rb:74