undefined method `size' for nil:NilClass

../chartutil.rb:15
../chartutil.rb:122:in `[]'
../chartutil.rb:122:in `time_hist_chart'
./google_chart/line_chart.rb:32:in `initialize'
../chartutil.rb:98:in `new'
../chartutil.rb:98:in `time_hist_chart'
/home/i/wrk/cia/site/index.cgi:61
/usr/lib/ruby/1.8/pstore.rb:322:in `transaction'
/usr/lib/ruby/1.8/pstore.rb:321:in `catch'
/usr/lib/ruby/1.8/pstore.rb:321:in `transaction'
/home/i/wrk/cia/site/index.cgi:57