can't convert nil into String (TypeError)

/usr/local/ruby/1.8/lib/ruby/1.8/erb.rb:419:in `initialize'
/usr/local/ruby/1.8/lib/ruby/1.8/erb.rb:419:in `new'
/usr/local/ruby/1.8/lib/ruby/1.8/erb.rb:419:in `scan'
/usr/local/ruby/1.8/lib/ruby/1.8/erb.rb:500:in `compile'
/usr/local/ruby/1.8/lib/ruby/1.8/erb.rb:668:in `initialize'
/home/harunaru/tdiary/tdiary.rb:209:in `new'
/home/harunaru/tdiary/tdiary.rb:209:in `do_eval_rhtml'
/home/harunaru/tdiary/tdiary.rb:162:in `eval_rhtml'
/home/harunaru/tdiary/image-gallery.rb:495
image-gallery.rb:2:in `require'
image-gallery.rb:2