attr_accessor issue

gistfile1.rb

class AnyOldClass
  attr_accessor :array_attr

  def initialize
    self.array_attr  = []
    this_is_fine

    self.array_attr  = []
    this_also_fine

    self.array_attr  = []
    this_throws_exception
  end

  def this_is_fine
    array_attr << 'foo'
  end

  def this_also_fine
    self.array_attr += ['foo']
  end

  def this_throws_exception
    # undefined method `+' for nil:NilClass (NoMethodError)
    array_attr += ['foo']
  end
end

AnyOldClass.new