Why doesn't sort or the spaceship (flying saucer) operator (<=>) work on booleans in Ruby?

In "Is it possible to sort a list of objects depending on if the individual object's response to a method?", I discovered that the flying saucer doesn't work on booleans.

Consider:

Ruby 1.8.7:

[true, false].sort # => undefined method `<=>' for true:TrueClass (NoMethodError)
true <=> false     # => undefined method `<=>' for true:TrueClass (NoMethodError)

Ruby 1.9.3:

[true, false].sort # => comparison of TrueClass with false failed (ArgumentError)
true <=> false     # => nil
true <=> true      # => 0
false <=> true     # => nil

It may have something to do with true and false not having a canonical sort order, because which comes first? But, that sounds pretty weak to me.

Is this a bug in sort?