Inheritance of BigDecimal breaks is_a? function in JRuby

gistfile1.rb

require "bigdecimal"

class Decimal < BigDecimal
end

# Inheritance breaks is_a? function
# Decimal.new("9876543210").is_a?(Decimal)
# => false
# jruby 1.6.7.2 (ruby-1.8.7-p357) (2012-05-01 26e08ba) (Java HotSpot(TM) 64-Bit Server VM 1.6.0_31) [darwin-x86_64-java]
puts "Decimal.new is a Decimal object?: #{Decimal.new("9876543210").is_a?(Decimal)}"

class SubString < String
end
# Inheriting from String works as expected.
puts "SubString.new is a SubString object?: #{SubString.new("test").is_a?(SubString)}"