Can't modify frozen Fixnum on Ruby 2.0 -

i have following code:

require 'prime' class numeric   #... math helpers    def divisors     return [self] if self == 1     @divisors ||= prime_division.map |n,p|       (0..p).map { |i| n**i }     end.inject([1]) |a,f|       a.product(f)     end.map { |f| f.flatten.reduce(:*) } - [self]   end    def divisors_sum      @divisors_sum ||= divisors.reduce(:+)   end     #... more methods evaluate code , 'caches' , assigns (||=) instance variables end 

wich outputs 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 remove caching instance variables @divisors, @divisors_sum... etc. , happens on ruby 2.0. ran on 1.9.3 without issues. what's happenning?

@divisors instance variable on instance of fixnum, trying alter it. shouldn't doing this.

what this?

module divisors   def self.for(number)     @divisors ||= { }     @divisors[number] ||= begin       case (number)       when 1         [ number ]       else         prime_division.map |n,p|           (0..p).map { |i| n**i }         end.inject([1]) |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 methods in numeric not modify instance, cache stored elsewhere.