Rails: Problems with parsing dates

I'm trying to save a record of a class being added to a schedule, then take the start date and add a week at a time adding additional entries into the database until I reach the end date. Here is my controller method

def schedule_class
  @program_schedule = ProgramSchedule.new(params[:program_schedule])

  if @program_schedule.save
    @program_schedule.schedule_classes
  end
end

So if I successfully save the model I call this method in the model

def schedule_classes
  class_start = Date.parse(self.first_date, '%m/%d/%Y')
  until class_start > self.last_date.to_date
  schedule = Schedule.new
    schedule.program_schedule_id = self.id
    schedule.start_date = class_start.to_s + ' ' + self.start_time
    schedule.end_date = class_start.to_s +' ' + self.end_time
    schedule.deleted = false

    schedule.save

    class_start.to_time.advance(:week => 1).to_date
  end
end

The error I get is $_ value need to be String (nil given)

I get that error on this line: class_start = Date.parse(self.first_date, '%m/%d/%Y')

And these are the parameters in program_schedule

{"commit"=>"Schedule Class",
  "authenticity_token"=>"MRtbXnPNq/xLr74awXfS4ksG/NK92aLwsogC8R8IHB0=",
  "utf8"=>"✓",
  "program_schedule"=>{"program_id"=>"1",
  "end_time"=>"01:00 am",
  "last_date"=>"08/25/2011",
  "first_date"=>"07/21/2011",
  "week_day_id"=>"5",
"start_time"=>"12:00 am"}}

Since the model has all it's values I'm not sure where the nil value error is coming from.