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

(plugin/archive.rb):53:in `navi_user_blog_category'
/virtual/202.212.68.246/cgi-def/admin/C-010/on/tdiary/tdiary.rb:72:in `join'
/virtual/202.212.68.246/cgi-def/admin/C-010/on/tdiary/tdiary.rb:72:in `safe'
/virtual/202.212.68.246/cgi-def/admin/C-010/on/tdiary/tdiary.rb:654:in `eval_src'
/virtual/202.212.68.246/cgi-def/admin/C-010/on/tdiary/tdiary.rb:901:in `do_eval_rhtml'
/virtual/202.212.68.246/cgi-def/admin/C-010/on/tdiary/tdiary.rb:851:in `eval_rhtml'
/virtual/202.212.68.246/cgi-def/admin/C-010/on/tdiary/index.rb:71