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

(erb):8:in `eval_rxml'
/usr/local/ruby-2.0/lib/ruby/2.0.0/erb.rb:849:in `eval'
/usr/local/ruby-2.0/lib/ruby/2.0.0/erb.rb:849:in `result'
/home/tdiary/users/kazuhiko/search.rb:79:in `eval_rxml'
/home/tdiary/users/kazuhiko/search.rb:255:in `<main>'