Ruby 1.9 % 操作符对于nil会抛异常
Ruby 1.9 测试：

# irb
irb(main):001:0> "%d" % nil
TypeError: can't convert nil into Integer
from (irb):1:in `%'
from (irb):1
from /bin/irb:12:in `<main>'

Ruby 1.8 不会抛出异常，默认会转换成0：

irb
irb(main):001:0> "%d" % nil
=> "0"