Debugging failures within method handle chain?

[Charles Oliver Nutter]

I'm at a loss for how to debug something like this:

MethodHandleNatives.java:336:in `raiseException':
java.lang.ClassCastException: required class org.jruby.RubyFixnum but
encountered class org.jruby.RubyObject
	from MethodHandleImpl.java:866:in `invoke_L5'
	from bench/bench_fib_recursive.rb:7:in `method__0$RUBY$fib_ruby'
	from MethodHandleImpl.java:866:in `invoke_L5'
	from bench/bench_fib_recursive.rb:7:in `method__0$RUBY$fib_ruby'

I assume I have something miswired, but since the MH doesn't print out
any information about what's being called it's hard for me to make a
determination about what I did wrong. How would I go about debugging a
failure in the chain that happens some time after I've wired it up and
bound it to a call site?

- Charlie