help with a NoMethodError caused by comparing start and end dates

I have a "job" model for adding job experiences to a resume and am
seeing the following error on occasion:

NoMethodError: undefined method `>' for nil:NilClass

The line of code throwing the error is: if self.started_on >
self.ended_on

The code for this portion of the job form view is:
  <label>Date started *</label>
    Start: <%= f.date_select :started_on, :order =>
[:month, :year], :start_year=>1970, :include_blank=>true %>
  <br/>
  <label>Date ended *</label>
    Stop: <%= f.date_select :ended_on, :order =>
[:month, :year], :start_year=>1970, :include_blank=>true %>
  <br>
  Hint: leave blank for current position

The relevant code from the job model is:
  def validate
    unless self.ended_on.nil?
      if self.started_on > self.ended_on
      errors.add_to_base( "Start date is after end date. Please check
your dates." )
      end
    end
  end

In the form, it tells the user to select a start date and end date,
but to leave the end date blank if they are still employed at that
job. I am not certain why this is generating an error sometimes but
not others, other than some combination of selecting the start and
stop dates in an unexpected way.

I have a "job" model for adding job experiences to a resume and am
seeing the following error on occasion:

NoMethodError: undefined method `>' for nil:NilClass

The line of code throwing the error is: if self.started_on >
self.ended_on

That means started_on is nil.

The code for this portion of the job form view is:
<label>Date started *</label>
Start: <%= f.date_select :started_on, :order =>
[:month, :year], :start_year=>1970, :include_blank=>true %>
<br/>
<label>Date ended *</label>
Stop: <%= f.date_select :ended_on, :order =>
[:month, :year], :start_year=>1970, :include_blank=>true %>
<br>
Hint: leave blank for current position

The relevant code from the job model is:
def validate
unless self.ended_on.nil?
if self.started_on > self.ended_on

I guess you have the situation where ended_on is not nil but
started_on is. If you don't think this should be happening then have
a look at the Rails Guide on debugging to find out how to use
ruby-debug to break into the code and inspect data.

Colin