Not sure if anyone actually use plan apply with proxy settings through PE, I think this is a possibly bug in all the last few puppet-agent rpms releases (6.27.1/6.28.0 & 7.21.0O) with

apply_ruby_shim.rb

The following error happens occurs due to the port never getting convert to a string,

Exited 1:
/opt/puppetlabs/puppet/lib/ruby/vendor_ruby/puppet/settings.rb:347:in `block in parse_global_options': undefined method `start_with?' for 4004:Integer (NoMethodError)
  from /opt/puppetlabs/puppet/lib/ruby/vendor_ruby/puppet/settings.rb:347:in `reject!'
  from /opt/puppetlabs/puppet/lib/ruby/vendor_ruby/puppet/settings.rb:347:in `parse_global_options'
  from /opt/puppetlabs/puppet/lib/ruby/vendor_ruby/puppet/settings.rb:308:in `initialize_global_settings'
  from /opt/puppetlabs/puppet/lib/ruby/vendor_ruby/puppet.rb:180:in `do_initialize_settings_for_run_mode'
  from /opt/puppetlabs/puppet/lib/ruby/vendor_ruby/puppet.rb:155:in `initialize_settings'
  from /opt/puppetlabs/puppet/bin/apply_ruby_shim.rb:81:in `<main>'

In apply_ruby_shim.rb:

val unless val.nil?

should probably be

<http://val.to|val.to>_s unless val.nil?

# Proxy will always be a string (even if its empty)
parsed_proxy = URI.parse(args['proxy'])
proxy_flags.each do |uri_method, setting_flag|
  val = parsed_proxy.send(uri_method)
  cli_settings_pluginsync << setting_flag << val unless val.nil?
end