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

(plugin/blog-category.rb):132:in `blog_category_entry'
/home/www/cgi-bin/diary/ns/diary03/tdiary.rb:75:in `join'
/home/www/cgi-bin/diary/ns/diary03/tdiary.rb:75:in `safe'
/home/www/cgi-bin/diary/ns/diary03/tdiary.rb:657:in `eval_src'
/home/www/cgi-bin/diary/ns/diary03/tdiary.rb:904:in `do_eval_rhtml'
/home/www/cgi-bin/diary/ns/diary03/tdiary.rb:854:in `eval_rhtml'
/home/www/cgi-bin/diary/ns/diary03/index.cgi:71