Rails Nested Model Forms Error

Question

Hi, I am trying to create a poll, which has many choices. While creating the poll, I am getting the "Poll can't be Blank" Error. Poll gets created when I removed the choices fields from the form

Poll.rb

class Poll < ActiveRecord::Base
  belongs_to :profile
  has_many :choices, :dependent => :destroy

  accepts_nested_attributes_for :choices, :reject_if => lambda {|a| a[:content].blank?} 

  validates_presence_of :question, :profile_id
  validates_numericality_of :profile_id
  validates_length_of :question, :in => 5..400
end

Choice.rb

class Choice < ActiveRecord::Base
  belongs_to :poll

  validates_presence_of :poll_id, :content
  validates_numericality_of :poll_id
  validates_length_of :content, :maximum => 300
end

PollsController

class PollsController < ApplicationController

  def new
    @poll = Poll.new
    4.times { @poll.choices.build}
  end

  def create
    @poll = Poll.new(params[:poll])
    @poll.profile_id = session[:user_id]
    if @poll.save
       flash[:notice] = "Poll is created"
       redirect_to poll_path(@poll)
    else
      render :controller => :polls, :action => 'new'
    end
  end
end

polls/new.html.erb

<% form_for (@poll) do |f|%>
<%=f.label :question%>
<p>
    <%=f.text_area :question, :rows => 5, :cols => 40%>
    <%=f.hidden_field :profile_id, :value => session[:user_id]%>
</p>
<% f.fields_for :choices do |builder|%>
    <p>
        <%=builder.label :content, "Choice"%>
        <%=builder.text_field :content%>
    </p>
<%end%>
<p>
    <%=f.submit "Create Poll"%>
</p>
<%end%>

Can somebody help me fix this problem. Thanks!!!