Can't convert TrueClass into String

After creating an executable using Warbler, I get the following error when running the JAR:
```
TypeError: can't convert TrueClass into String
   (root) at file:/[path]/app/app.jar!/META-INF/init.rb:14
  require at org/jruby/RubyKernel.java:1065
   (root) at file:/var/folders/f2/9k757ht96kndfnvlmfhx5bp00000gn/T/jruby3396284761916552030extract/jruby-complete-1.7.12.jar!/META-INF/jruby.home/lib/ruby/shared/rubygems/core_ext/kernel_require.rb:1
  require at file:/var/folders/f2/9k757ht96kndfnvlmfhx5bp00000gn/T/jruby3396284761916552030extract/jruby-complete-1.7.12.jar!/META-INF/jruby.home/lib/ruby/shared/rubygems/core_ext/kernel_require.rb:55
error: org.jruby.embed.EvalFailedException: (TypeError) can't convert TrueClass into String
```

This is the contents of `META-INF/init.rb` from the extracted JAR:

```
WARBLER_CONFIG = {}


ENV['GEM_HOME'] = File.expand_path('../..', __FILE__)


ENV['BUNDLE_GEMFILE'] = File.expand_path('../../app/Gemfile', __FILE__)

ENV['BUNDLE_WITHOUT'] = 'development:test:assets'

$LOAD_PATH.unshift __FILE__.sub(/!.*/, '!/app/lib')
require 'rubygems'
# the jars are already loaded via the servlet classloader
ENV['JBUNDLE_SKIP'] = true
```

I'm not sure why `ENV['JBUNDLE_SKIP'] = true` would cause this error. I am using jbundler to include JAR dependencies.