Can't modify frozen Fixnum on Ruby 2.0

I have the following code:

require 'prime'
class Numeric
  #... math helpers

  def divisors
    return [self] if self == 1
    @divisors ||= prime_division.map do |n,p|
      (0..p).map { |i| n**i }
    end.inject([1]) do |a,f|
      a.product(f)
    end.map { |f| f.flatten.reduce(:*) } - [self]
  end

  def divisors_sum
     @divisors_sum ||= divisors.reduce(:+)
  end

   #... more methods that evaluate code and 'caches' and assigns (||=) to instance variables
end

Which outputs an error with:

> 4.divisors
/home/cygnus/Projects/project-euler/stuff/numbers.rb:24:in `divisors_sum': can't modify frozen Fixnum (RuntimeError)

The error disappears when I remove caching into the instance variables @divisors, @divisors_sum... etc. And this only happens on ruby 2.0. Ran it on 1.9.3 without issues. What's happening?

@divisors is an instance variable on an instance of Fixnum, so you are trying to alter it. You probably shouldn't be doing this.

What about this?

module Divisors
  def self.for(number)
    @divisors ||= { }
    @divisors[number] ||= begin
      case (number)
      when 1
        [ number ]
      else
        prime_division.map do |n,p|
          (0..p).map { |i| n**i }
        end.inject([1]) do |a,f|
          a.product(f)
        end.map { |f| f.flatten.reduce(:*) } - [ number ]
      end
    end
  end

  def self.sum(number)
     @divisors_sum ||= { }
     @divisors_sum[number] ||= divisors(number).reduce(:+)
  end
end

class Numeric
  #... math helpers

  def divisors
    Divisors.for(self)
  end

  def divisors_sum
     Divisors.sum(self)
  end
end

This means that the methods in Numeric do not modify any instance, the cache is stored elsewhere.