jruby 9.0.4.0 (2.2.2) 2015-11-17 b9fb7aa Java HotSpot(TM) 64-Bit Server VM 25.66-b17 on 1.8.0_66-b17 +jit [linux-amd64]

When running our RSpec tests under JRuby 9k, some tests emit the following warning:

setEvalType unimplemented in org.jruby.RubySymbol$1

I haven't yet been able to pinpoint what is actually leading to setEvalType being called as it seems to be impossible to monkey-patch org.jruby.runtime.BlockBody where this method is defined (monkey-patching seems to have no effect at all). Any hints how to effectively debug this are much appreciated!