Strange behaviour of the Integer kernel method.

integer_cast_error.rb

irb(main):002:0> Integer("01")
=> 1
irb(main):003:0> Integer("02")
=> 2
irb(main):004:0> Integer("03")
=> 3
irb(main):005:0> Integer("05")
=> 5
irb(main):006:0> Integer("06")
=> 6
irb(main):007:0> Integer("07")
=> 7
irb(main):008:0> Integer("08")
ArgumentError: invalid value for Integer: "08"
        from (irb):8:in `Integer'
        from (irb):8