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.