assert2 0.3.2 and ActiveSupport 2.1.2

I've found a conflict between assert2 and ActiveSupport. Both define the
method in_groups_of() with different signatures and semantics. It shows up in
Ultrasphinx when I run my tests. I've reproduced it in the console, see
below. Any suggests on how to force the desired in_groups_of() method?

TIA,
  Jeffrey

script/console

Loading development environment (Rails 2.1.2)

a = [1, 2, 3, 4]

=> [1, 2, 3, 4]

a.in_groups_of(2)

=> [[1, 2], [3, 4]]

require 'assert2'

=> ["Assert_2_0"]

a.in_groups_of(2)

LocalJumpError: no block given
  from /usr/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in `each_slice'
  from /usr/lib/ruby/gems/1.8/gems/assert2-0.3.1/lib/ruby_reflector.rb:829:in `each'
  from /usr/lib/ruby/gems/1.8/gems/assert2-0.3.1/lib/ruby_reflector.rb:829:in `each_slice'
  from /usr/lib/ruby/gems/1.8/gems/assert2-0.3.1/lib/ruby_reflector.rb:829:in `in_groups_of'
  from (irb):4