Form errors not showing in Ruby on Rails

I'm trying to create a registration form with Rails. It's working, but it doesn't displays the errors from the validation (it validates, but the errors don't display).

Here are my files:

# new.html.erb
<h1>New user</h1>

<% form_for :user, :url =>{:action=>"new", :controller=>"users"} do |f| %>
  <%= f.error_messages %>

  <p>
    <%= f.label :name %><br />
    <%= f.text_field :name %>
  </p>
  <p>
    <%= f.label :password %><br />
    <%= f.password_field :password %>
  </p>
  <p>
    <%= f.submit 'Create' %>
  </p>
<% end %>

<%= link_to 'Back', users_path %>

# user.rb
class User < ActiveRecord::Base
    validates_presence_of :name
    validates_presence_of :password
end

#users_controller.rb
class UsersController < ApplicationController

    def index
        @users = User.all
    end


    def show
        @user = User.find(params[:id])
    end

    def new
        if session[:user_id].nil?           
            if params[:user].nil? #User hasn't filled the form
                @user = User.new
            else #User has filled the form
                user = User.new(params[:user])

                if user.save
                    user.salt = rand(1000000000)
                    user.password = Digest::MD5.hexdigest(user.salt.to_s + user.password)
                    user.save
                    flash[:notice] = 'User was successfully created.'
                    session[:user_id] = user.id
                    session[:password] = user.password
                    redirect_to url_for(:action=>"index",:controller=>"users")
                else
                    render :action=>"new"
                end
            end

        else #User is already logged in
            flash[:notice] = 'You are already registered.'
            redirect_to url_for(:action=>"index")
        end
     end 

# some other actions removed....


end

Why aren't the errors being displayed?

Thanks!!