Is this an intended change?

In ruby 1.6.7, Array#sort tolerates unordered objects:

$ ~/ruby/src/ruby-1.6.7/ruby
class C; end
class C1 < C; end
class C2 < C; end
p [C, C1, C2].sort
[C2, C1, C]

In 1.7.3, it doesn't:

$ ruby
class C; end
class C1 < C; end
class C2 < C; end
p [C, C1, C2].sort
-:4:in `sort': undefined method `>' for nil (NoMethodError)
         from -:4