class Dog
  attr_accessor :name, :breed, :age
  @@num_of_dogs=0
  def initialize(name, breed, age)
    @name=name
    @breed=breed
    @age=age
    puts "dog number: #{@@num_of_dogs} has been created.
    Name: #{@name}
    Breed: #{@breed}
    Age: #{@age}"
    @@num_of_dogs+=1
  end
  def bark(s)
    puts s
  end
  def bark
    puts "waff waff!! I'm a dog"
  end
end

require "dog.rb"
class Collie < Dog
  attr_accessor :color
  @@num_of_collies=0
  def initialize(s, a, n)
    @color=s
    super(n,"collie", a)
    puts "color: #{@color}"
  end
  def bark
     if color == "brown"
      super.bark    ***********************************************
     else
       puts "waff waff!! i'm a collie"
     end
  end
end

require "collie.rb"
cc=Collie.new("gold", 74, "bobik")
cc.bark
c=Collie.new("brown", 2, "palkan")
c.bark


./collie.rb:12:in `bark': undefined method `bark' for nil:NilClass (NoMethodError)
    from stack.rb:7
dog number: 0 has been created.
    Name: bobik
    Breed: collie
    Age: 74
color: gold
waff waff!! i'm a collie
dog number: 1 has been created.
    Name: palkan
    Breed: collie
    Age: 2
color: brown
waff waff!! I'm a dog

can anyone tell me what's the error about?
I marked the line 12 in collie.rb with ****************************