rake aborted! uninitialized constant Encoding ubuntu

I am new in Ruby on Rails, I am working on ubuntu10.04

I get this error when i run rake db:migrate

  (in /home/mostafa/Desktop/book_tracker)
  rake aborted!
  uninitialized constant Encoding

  (See full trace by running task with --trace)

Hope Someone can help in such matter

Hi there,

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