Caching RVM installs fails for jruby

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When following the guide here: https://circleci.com/docs/1.0/rvm-cache-ruby-binaries/

And trying to cache jruby-9.1.8.0 into ~/rvm_binaries

I get this error:

ppt_ruby_version=jruby-9.1.8.0
if [[ -e ~/rvm_binaries/$ppt_ruby_version.tar.gz ]]
then
  rvm mount ~/rvm_binaries/$ppt_ruby_version.tar.gz
else
  mkdir -p ~/rvm_binaries
  rvm install $ppt_ruby_version
  cd ~/rvm_binaries && rvm prepare $ppt_ruby_version
fi

The given paths '/home/ubuntu/rvm_binaries/jruby-9.1.8.0.tar.gz' do not point to a ruby installation.

Any ideas?