在看 Programming Ruby（ruby 2.0 版）, 有下面一段代码，一直运行报错，提示enusre 关键字无法使用。不知道为什么会这样？

我的 ruby 版本是 2.1，切换到 2.0 同样是报错

• 代码：

rate_mutex = Mutex.new
exchange_rates = ExchangeRates.new
exchange_rates.update_from_online_feed

Thread.new do
  rate_mutex.lock
  loop do
    rate_mutex.sleep 3600
    exchange_rates.update_from_online_feed
  end
end

loop do
  print "Enter currency code and amount: "
  line = gets
  if rate_mutex.try_lock
    puts(exchange_rates.convert(line)) ensure rate_mutex.unlock
  else
    puts "Sorry, rates being updated. Try again in a minute"
  end
end

• 报错信息： ➜ prog_ruby ruby multiple_threads.rb multiple_threads.rb:17: syntax error, unexpected keyword_ensure, expecting keyword_end puts(exchange_rates.convert(line)) ensure rate_mutex.unlock ^