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

(plugin/archive.rb):61:in `navi_user_blog_category'
/home/www/cgi-bin/diary/jc/diary09/tdiary.rb:75:in `join'
/home/www/cgi-bin/diary/jc/diary09/tdiary.rb:75:in `safe'
/home/www/cgi-bin/diary/jc/diary09/tdiary.rb:657:in `eval_src'
/home/www/cgi-bin/diary/jc/diary09/tdiary.rb:904:in `do_eval_rhtml'
/home/www/cgi-bin/diary/jc/diary09/tdiary.rb:854:in `eval_rhtml'
/home/www/cgi-bin/diary/jc/diary09/index.cgi:71