Rails 3 bug: undefined method `<=>' for class `ActiveSupport::Multibyte::Chars'

I'm getting the following bug when I try to run rake db:migrate in
Rails 3 beta 4:

$ rake db:migrate --trace
(in C:/Users/Adam/Documents/Aptana Workspace/QuestionBank)
default formats are encoded in ISO-8859-1
** Invoke db:migrate (first_time)
** Invoke environment (first_time)
** Execute environment
** Execute db:migrate
== CreateUsers: migrating

An update on this - it seems to work if the line "undef <=>" is
removed from ActiveSupport::Multibyte::Chars