ActiveResource 3 on MRI 191: NameError (uninitialized constant Builder::XmlBase::Symbol)

We've run into this before and, in this particular project, I'm
positive we had a hack somewhere to get AR to work but it seems to
have gone missing and I'm not finding any hints in git history or
Google. Any call to an AR.create will trigger it. Seems like a
strategically placed require or include did the trick but I'm probably
misremembering.

The offending code in Builder is xmlbase:40:
sym = "#{sym}:#{args.shift}" if args.first.kind_of?(Symbol)

Why that method_missing is firing off xmlmarkup:239 is a bit of a
mystery to me, as 239 is just:
_ensure_no_block block_given?

the backtrace:

builder (2.1.2) lib/builder/xmlbase.rb:40:in `method_missing'
builder (2.1.2) lib/builder/xmlmarkup.rb:239:in `instruct!'
relock/gems/activesupport-3.0.0.beta3/lib/active_support/core_ext/hash/
conversions.rb:136:in `to_xml'
[...]

Any suggestions on stuff to try or look into?