Ruby - Recursive stack level too deep

class Node 
   
  #data = nil 
  #children = nil 
   
  attr_accessor :children, :data 
     
  def initialize(data, children=Array.new) 
    self.data = data 
    self.children = children
  end 
   
  def addchild(child) 
    self.children << child 
  end 
   
  def printtree() 
    print self.data, "\n" 
    self.children.each{ |child|  
      print child.printtree() 
      } 
  end   
end

n1 = Node.new('n1')
n2 = Node.new('n2')
n3 = Node.new('n3', [n2, n1])

#n2.printtree
n3.printtree