undefined method `[]' for nil (NameError)

(plugin/amazon.rb):167:in `amazon_get'
/home/ryu-m/public_html/tdiary.rb:75:in `join'
/home/ryu-m/public_html/tdiary.rb:75:in `safe'
/home/ryu-m/public_html/tdiary.rb:659:in `eval_src'
/home/ryu-m/public_html/tdiary.rb:904:in `do_eval_rhtml'
/home/ryu-m/public_html/tdiary.rb:854:in `eval_rhtml'
/home/ryu-m/public_html/index.rb:71