rails redirect_to :back not working

I'm trying to use the following:

class PaymentsController < ApplicationController

  def addproduct
     (session[:products] ||= []) << params[:item]
     redirect_to :back
  end

 end

I got this exception:

undefined method `back_url' for #<PaymentsController:0x007ff682c467a8>

Why this is happening?