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