what happen? when I update rails from 1.2.6 to 2.0.2

/!\ FAILSAFE /!\ Thu Jan 03 13:49:24 -0800 2008
  Status: 500 Internal Server Error
  You have a nil object when you didn't expect it!
You might have expected an instance of ActiveRecord::Base.
The error occurred while evaluating nil.[]
    c:/usr/ruby/lib/ruby/1.8/cgi.rb:1166:in `[]'

Today I update rails from 1.2.6 to 2.0.2. I run ruby script/server
WEBrick
I got this error. what happen.. Who can help me? Thanks!
by the way, I run this app is correct under rails 1.2.6

o, the first error is
NoMethodError (You have a nil object when you didn't expect it!
You might have expected an instance of ActiveRecord::Base.
The error occurred while evaluating nil.[]):
    c:/usr/ruby/lib/ruby/1.8/cgi.rb:1166:in `[]'

I run rails in windows XP.