Bundler issue #2915: 'builtin_gem?': undefined method 'include?' for nil:NilClass

bundler_env

Bundler 1.6.0.rc2
Ruby 2.1.1 (2014-02-24 patchlevel 76) [x86_64-darwin12.0]
Rubygems 2.2.2
GEM_HOME /Users/thetamind/.gem/ruby/2.1.1
GEM_PATH /Users/thetamind/.gem/ruby/2.1.1:/Users/thetamind/.rubies/ruby-2.1.1/lib/ruby/gems/2.1.0

Bundler settings
  jobs
    Set for your local app (/private/tmp/repro-2915/.bundle/config): "4"
  path
    Set for your local app (/private/tmp/repro-2915/.bundle/config): "vendor"
  disable_shared_gems
    Set for your local app (/private/tmp/repro-2915/.bundle/config): "1"


Gemfile
source "https://rubygems.org"

gem 'activesupport', '4.0.4'
gem 'simple-navigation', '3.13.0'


Gemfile.lock
GEM
  remote: https://rubygems.org/
  specs:
    activesupport (4.0.4)
      i18n (~> 0.6, >= 0.6.9)
      minitest (~> 4.2)
      multi_json (~> 1.3)
      thread_safe (~> 0.1)
      tzinfo (~> 0.3.37)
    atomic (1.1.16)
    i18n (0.6.9)
    minitest (4.7.5)
    multi_json (1.9.2)
    simple-navigation (3.13.0)
      activesupport (>= 2.3.2)
    thread_safe (0.3.1)
      atomic (>= 1.1.7, < 2)
    tzinfo (0.3.39)

PLATFORMS
  ruby

DEPENDENCIES
  activesupport (= 4.0.4)
  simple-navigation (= 3.13.0)

repro-2915.sh

#!/bin/bash

set -ex

ISSUE=2915
mkdir -p /tmp/repro-$ISSUE
cd /tmp/repro-$ISSUE

cat > Gemfile <<RUBY
source "https://rubygems.org"

gem 'activesupport', '4.0.4'
RUBY

# Should report "Using minitest 4.7.5" on ruby-2.1.1
bundle install --path vendor --jobs 4

bundle cache

echo "gem 'simple-navigation', '3.13.0'" >> Gemfile

# Exception on first run
bundle install --jobs 4 || echo -e "Exception on first run\n"

# Works correctly on second run
bundle install --jobs 4

# Removing spec triggers exception again
rm vendor/ruby/*/specifications/simple-navigation-3.13.0.gemspec
bundle install --jobs 4 || echo -e "Removing spec triggers exception again\n"

# Works correctly with jobs=1
rm vendor/ruby/*/specifications/simple-navigation-3.13.0.gemspec
bundle install --jobs 1 && echo -e "Works correctly with jobs=1\n"