undefined method `parse' for String:Class

Hi,

I use a script called : add_admin.rake
When i execute this : 'rake -s -f Rakefile mailserv:add_admin --trace
i have this error :
** Invoke mailserv:add_admin (first_time)
** Invoke environment (first_time)
** Execute environment
** Execute mailserv:add_admin
rake aborted!
undefined method `parse' for String:Class
/usr/local/lib/ruby/gems/1.8/gems/highline-1.6.9/lib/highline/question.rb:329:in
`convert'
/usr/local/lib/ruby/gems/1.8/gems/highline-1.6.9/lib/highline.rb:268:in
`ask'
/var/mailserv/admin/lib/tasks/add_admin.rake:36
/usr/local/lib/ruby/gems/1.8/gems/rake-0.9.2.2/lib/rake/task.rb:205:in
`call'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.9.2.2/lib/rake/task.rb:205:in
`execute'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.9.2.2/lib/rake/task.rb:200:in
`each'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.9.2.2/lib/rake/task.rb:200:in
`execute'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.9.2.2/lib/rake/task.rb:158:in
`invoke_with_call_chain'
/usr/local/lib/ruby/1.8/monitor.rb:242:in `synchronize'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.9.2.2/lib/rake/task.rb:151:in
`invoke_with_call_chain'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.9.2.2/lib/rake/task.rb:144:in
`invoke'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.9.2.2/lib/rake/application.rb:116:in
`invoke_task'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.9.2.2/lib/rake/application.rb:94:in
`top_level'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.9.2.2/lib/rake/application.rb:94:in
`each'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.9.2.2/lib/rake/application.rb:94:in
`top_level'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.9.2.2/lib/rake/application.rb:133:in
`standard_exception_handling'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.9.2.2/lib/rake/application.rb:88:in
`top_level'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.9.2.2/lib/rake/application.rb:66:in
`run'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.9.2.2/lib/rake/application.rb:133:in
`standard_exception_handling'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.9.2.2/lib/rake/application.rb:63:in
`run'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.9.2.2/bin/rake:33
/usr/local/bin/rake:19:in `load'
/usr/local/bin/rake:19
Tasks: TOP => mailserv:add_admin

Any idea ?

'gem env' give me :
RubyGems Environment:
  - RUBYGEMS VERSION: 1.8.10
  - RUBY VERSION: 1.8.7 (2011-12-28 patchlevel 357) [i386-openbsd]
  - INSTALLATION DIRECTORY: /usr/local/lib/ruby/gems/1.8
  - RUBY EXECUTABLE: /usr/local/bin/ruby18
  - EXECUTABLE DIRECTORY: /usr/local/bin
  - RUBYGEMS PLATFORMS:
    - ruby
    - x86-unknown
  - GEM PATHS:
     - /usr/local/lib/ruby/gems/1.8
     - /root/.gem/ruby/1.8
  - GEM CONFIGURATION:
     - :update_sources => true
     - :verbose => true
     - :benchmark => false
     - :backtrace => false
     - :bulk_threshold => 1000
  - REMOTE SOURCES:
     - http://rubygems.org/

'gem list rails' give me :
rails (3.1.3, 2.3.4)

the file add_admin.rake :
namespace :mailserv do
  def ask_passwords
    pass1 = ask("Password: ") do |q|
      q.echo = "*"
      q.validate = /^.{6,31}$/
      q.responses[:not_valid] = "Please use a stronger password (min 6
characters)"
    end
    pass2 = ask("Password Confirm: ") {|q| q.echo = "*" }
    while pass1 != pass2
      say "\nPasswords don't match"
      pass1 = ask("Password: ") do |q|
        q.echo = "*"
        q.validate = /^.{6,31}$/
        q.responses[:not_valid] = "Please use a stronger password (min 6
characters)"
      end
      pass2 = ask("Password Confirm: ") {|q| q.echo = "*" }
    end
    return pass1
end
  # Alias for add_admin
  task :create_admin do
    Rake::Task['mailserv:add_admin'].execute
  end
  desc "Create a new user."
  task :add_admin => :environment do
    require 'highline/import'
    say "Add an Administrator to the system"
    begin
      begin
        username = ask("username: ", String) do |q|
          q.validate = /^([a-zA-Z0-9\_\-]+)$/
          q.responses[:not_valid] = "Please use characters 'a-z, A-Z,
0-9, _-' only."
        end
        email = ask("E-mail (used to send status updates - not needed):
")
        password = ask_passwords
end while !agree("Is this correct? ", true)
      admin = Admin.new(:username => username, :email => email,
        :password => password)
      unless admin.save
        say "\n" + admin.errors.full_messages.join(", ") + "\n\n"
      end
    end while !admin.errors.size.zero?
  end
end

Thank you very much for your help.

i find the solution update Highline using : gem install highline