Web server won't start after freezing gems

My hosting company recently upgraded their Rails version, so I want to
freeze my app to Rails version 1.2.3. I was able to freeze it
successfully, but when I try to start the server on my local
development environment, I'm confronted with this error:

./script/../config/../vendor/rails/railties/lib/initializer.rb:43:in
`send': undefined method `install_gem_spec_stubs' for
#<Rails::Initializer:0x290444c> (NoMethodError)
  from ./script/../config/../vendor/rails/railties/lib/initializer.rb:
43:in `run'
  from ./script/../config/boot.rb:46:in `load_initializer'
  from ./script/../config/boot.rb:38:in `run'
  from ./script/../config/boot.rb:11:in `boot!'
  from ./script/../config/boot.rb:109
  from ./script/server:2:in `require'
  from ./script/server:2
  from -e:4:in `load'
  from -e:4

Does anyone know what I can do to resolve this? Thanks.