How to use csv to open file in gbk code

there is an file(/home/pt/test/shbase.csv) in gbk not utf-8,the
following is my progrmm:
require 'csv'
CSV.open('/home/pt/test/shbase.csv', 'r') do |row|
while line=row.gets
puts line
end
end
wrong message:
/usr/local/ruby/lib/ruby/1.9.1/csv.rb:1988:in `=~': invalid byte
sequence in UTF-8 (ArgumentError)
how to open it use csv?

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