Unable to push app to heroku... env: bundle: No such file or directory

Have had an app up and working for some time, but now I cannot git push:

-----> Ruby app detected
-----> Compiling Ruby/Rails
-----> Using Ruby version: ruby-1.9.3-p125
-----> Installing dependencies using 
       Fixing nokogiri install. Clearing bundler cache.
       See https://github.com/sparklemotion/nokogiri/issues/923.
       Running: bundle install --without development:test --path vendor/bundle --binstubs vendor/bundle/bin --deployment
       env: bundle: No such file or directory
       Bundler Output: env: bundle: No such file or directory
 !
 !     Failed to install gems via Bundler.
 !

 !     Push rejected, failed to compile Ruby app

I checked the path per https://github.com/heroku/heroku-buildpack-ruby/commit/00ae3eb09522028b692bfda15d4089b5531f0b79 and heroku config reports:

GEM_PATH:                   vendor/bundle/ruby/1.9.1
LANG:                       en_US.UTF-8
PATH:                       bin:vendor/bundle/ruby/1.9.1/bin:/usr/local/bin:/usr/bin:/bin

so it looks like the path is correct, although I don't understand why it's ruby 1.9.1 when I'm using 1.9.3.

Gemfile contains:

source 'https://rubygems.org'

gem 'rails', '~>3.2'

FWIW I have tried a bundle update and comitted the change to gemfile.lock, but still unable to push it.