Redmine

I am trying to import some data from our own defect tracker into Redmine. I am trying to create a Rake task but I very new to Rake tasks. I am getting the following error:

rake redmine:migrate_from_merant_tracker RAILS_ENV="production" 
(in /Sites/redmine)
Attempting Migration from Merant Tracker
Migrating ticketsrake aborted!
uninitialized constant Issue
/Sites/redmine/rakefile:10
(See full trace by running task with --trace)

Here is my code:

puts "Attempting Migration from Merant Tracker" 
namespace :redmine do
  desc 'Merant Tracker migration script'
  task :migrate_from_merant_tracker => :environment do
    CSV.open('Sites/redmine/lib/tasks/issues.csv', 'r') do |row|
        @tempMT = MerantTracker.new(row[0],row[1],row[2],row[3],row[4],row[5],row[6],row[7],row[8])  
        @i = Issue.new :project => Project.find_by_id(1),
                      :start_date => "2000-01-01",
                      :subject => @tempMT.subject,
                      :description => @tempMT.input_id,
                      :tracker => find_by_id(1),
                      :status => IssuesStatus.find_by_id(5),
                      :author => Users.find(3),
                      :priority => Enumeration.find_by_id(3)
        @i.save

    end
  end
end

Can anyone help? Much appreciated. We want to use redmine at work but would like to be able to import some of our old data.

Paul