500 Internal Server Error

getting title was failed (RuntimeError)

(plugin/ref_tools.rb):91:in `get_title'
./tdiary.rb:110:in `join'
./tdiary.rb:110:in `safe'
./tdiary.rb:801:in `eval_src'
./tdiary.rb:1152:in `do_eval_rhtml'
./tdiary.rb:1089:in `eval_rhtml'
./tdiary.rb:1780:in `eval_rhtml'
index.rb:81