rbx: Bignum#<=> doesn't do something right... coersion?

gistfile1.txt

In ruby-1.8.7

C:\sam>irb
irb(main):001:0> class A
irb(main):002:1>   @a
irb(main):003:1>   def initialize(a)
irb(main):004:2>     @a=a
irb(main):005:2>   end
irb(main):006:1>   def coerce(other)
irb(main):007:2>     if other.is_a? Integer
irb(main):008:3>       [A.new(other), self]
irb(main):009:3>     else
irb(main):010:3*       nil
irb(main):011:3>     end
irb(main):012:2>   end
irb(main):013:1>   def <=>(other)
irb(main):014:2>     @a <=> other.to_i
irb(main):015:2>   end
irb(main):016:1>   def to_i
irb(main):017:2>     @a.to_i
irb(main):018:2>   end
irb(main):019:1> end
=> nil
irb(main):020:0> five = A.new(5)
=> #<A:0x134a220 @a=5>
irb(main):021:0> (2**64) <=> five
=> 1

In rbx-1.1.0 (or HEAD as of Nov 17, 2010), the last line returns nil instead.