Problems with setup.rb and metaconfig

I’m doing some fiddling around with a setup.rb package, and wanted to
be able to pass in another invocation time option, a la ruby setup.rb
–option. The docs seem to suggest using metaconfig to add this, so I
rolled in a metaconfig file into the package root that looked like this:

add_path_config ‘gem_install_root’, ‘’, ‘Path to root of build

When setup.rb gets called (from a make process) I get a nasty error
about frozen arrays:

/usr/bin/ruby1.8 setup.rb config --siterubyver=/usr/lib/ruby/1.8
setup.rb:100:in push': can't modify frozen array (TypeError) from setup.rb:100:inadd’
from setup.rb:508:in add_path_config' from /home/matt/sandbox/packages/libgems-ruby-0.9.4/ metaconfig:1:inload_script’
from setup.rb:831:in run_metaconfigs' from setup.rb:807:ininvoke’
from setup.rb:773:in `invoke’
from setup.rb:1578

Am I doing something dumb here? I’d be grateful for any pointers…