RSPEC and Custom Validation date fields

HI All,
I have a model that uses custom validation. The validation works from
the browser but RSPEC gives it an error. It looks to me like RSPEC
isn't seeing the start_time and end_time fields in my model. If I
change line 9 in my validation to something like "unless title ==
title" I don't have any problem. The issues seems to be with date
fields. I suspect this is a bug but since I'm new to RSPEC I'd like
advice before reporting it.

Here is the error from rspec:

    You have a nil object when you didn't expect it!
    The error occurred while evaluating nil.<

    /home/leo/railsprojects/trunk/memorymap/app/models/event.rb:9:in
`valid_dates?'
    /home/leo/railsprojects/trunk/memorymap/spec/models/event_spec.rb:
24:

    7
    8 def valid_dates?
    9 unless start_time < end_time
    10 errors.add("End Time must be later than Start Time")
    11 end

Here is my model:

1 class Event < ActiveRecord::Base
  2
validates_presence_of :title, :description, :start_time, :end_time
  3 validates_uniqueness_of :title
  4 validate_on_create :valid_dates?
  5
  6 private
  7
  8 def valid_dates?
  9 unless start_time < end_time
10 errors.add("End Time must be later than Start Time")
11 end
12 end
13 end