any ideas what's up with this? I'm running into it too, trying to use ruby-debug, I actually get it just trying to step through my code with next, not explicitly doing a bt. Which of course is making it hard to debug what I'm really trying to debug!
I'm not totally sure of the history here, but it seems like DynamicScope.getEvalScope() must have changed cardinality at some point in the past. In the latest version of jruby-debug there's some code to determine whether the runtime parameter is needed or not: