gem install rails gives me Error: undefined method include?

I'm using ubuntu 9.10, apache 2 and mysql 5. I install ruby 1.8.7 on the server. I ran the command

gem install rails -v 1.2.3

But then I get the error

ERROR:  While executing gem ... (NoMethodError)
    undefined method `include?' for nil:NilClass

What did I do wrong?

From serverfault John

• I believe you need:

  gem install rails -v=1.2.3
  

  or gem will think that 1.2.3 is a new gem to install, add the = sign.

  From andre

• I formatted the disk and installed everything all over again. I was trying to replicate the settings of a very old ruby on rails server. So this time I made sure I had the correct version of all the old dependent gems. This seemed to have solved "some" of the problems.

  From John