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

(plugin/amazon.rb):145:in `get_amazon_image'
/virtual/truelove/public_html/blog1.masaomix.com/tdiary.rb:75:in `join'
/virtual/truelove/public_html/blog1.masaomix.com/tdiary.rb:75:in `safe'
/virtual/truelove/public_html/blog1.masaomix.com/tdiary.rb:657:in `eval_src'
/virtual/truelove/public_html/blog1.masaomix.com/tdiary.rb:904:in `do_eval_rhtml'
/virtual/truelove/public_html/blog1.masaomix.com/tdiary.rb:854:in `eval_rhtml'
/virtual/truelove/public_html/blog1.masaomix.com/index.rb:71