Error Simple Login “undefined method `key?' for nil:NilClass”-Collection of common programming errors

Please help-me i wen make a simple login and this error appear when i test this. Please how i solve this error?

NoMethodError in UserController#login

undefined method `key?’ for nil:NilClass

  def login
    if request.post?
      if session[:user] = User.authenticate(params[:user][:login],params[:user][:password])
        flash[:message] = 'Voce esta Logado !'
        redirect_to_stored
      else
        flash[:warning] = 'Usuario e/ou Senha incorreta !'
      end
    end
  end

Originally posted 2013-11-09 20:51:30.