"defined super" causes NullPointerException in mixins

class A
  def x
    puts "A"
  end
end
module B
  def x
    puts "B"
    super() if defined? super()
  end
end
class C < A
  include B
  def x
    puts "C"
    super()
  end
end

C.new.x
__END__
Expected output:
C
B
A

Real output
C
B

With --1.8

bug.rb:-1:in `ensure_1$RUBY$__ensure__': java.lang.NullPointerException
	from bug.rb:9:in `method__3$RUBY$x'
	from bugInvokermethod__3$RUBY$xFixed0#x:65535:in `call'
	from SuperCallSite.java:310:in `cacheAndCall'
	from SuperCallSite.java:110:in `callBlock'
	from SuperCallSite.java:115:in `call'
	from bug.rb:16:in `method__5$RUBY$x'
	from bugInvokermethod__5$RUBY$xFixed0#x:65535:in `call'
	from bugInvokermethod__5$RUBY$xFixed0#x:65535:in `call'
	from CachingCallSite.java:289:in `cacheAndCall'
	from CachingCallSite.java:108:in `call'
	from bug.rb:20:in `__file__'
	from bug.rb:-1:in `load'
	from Ruby.java:685:in `runScript'
	from Ruby.java:568:in `runNormally'
	from Ruby.java:414:in `runFromMain'
	from Main.java:286:in `run'
	from Main.java:128:in `run'
	from Main.java:97:in `main'
  
With --1.9
  
SuperNode.java:119:in `definition': java.lang.NullPointerException
	from DefinedNode.java:86:in `interpret'
	from IfNode.java:111:in `interpret'
	from NewlineNode.java:104:in `interpret'
	from BlockNode.java:71:in `interpret'
	from InterpretedMethod.java:113:in `call'
	from SuperCallSite.java:280:in `cacheAndCall'
	from SuperCallSite.java:64:in `callBlock'
	from SuperCallSite.java:69:in `call'
	from SuperNode.java:111:in `interpret'
	from NewlineNode.java:104:in `interpret'
	from BlockNode.java:71:in `interpret'
	from InterpretedMethod.java:139:in `call'
	from CachingCallSite.java:289:in `cacheAndCall'
	from CachingCallSite.java:108:in `call'
	from CallNoArgNode.java:61:in `interpret'
	from NewlineNode.java:104:in `interpret'
	from BlockNode.java:71:in `interpret'
	from RootNode.java:129:in `interpret'
	from Ruby.java:711:in `runInterpreter'
	from Ruby.java:572:in `runNormally'
	from Ruby.java:414:in `runFromMain'
	from Main.java:286:in `run'
	from Main.java:128:in `run'
	from Main.java:97:in `main'