undefined method `[]' for nil:NilClass (NoMethodError)

(plugin/amazon.rb):141:in `get_amazon_image'
./tdiary.rb:75:in `join'
./tdiary.rb:75:in `safe'
./tdiary.rb:657:in `eval_src'
./tdiary.rb:904:in `do_eval_rhtml'
./tdiary.rb:854:in `eval_rhtml'
index.cgi:71