Error trying to archive with xcode4_archive_mode = true

gistfile1.txt

** Invoke version (first_time)
** Execute version
Setting version to 0.3.0.854
** Invoke tag (first_time)
** Execute tag
Tagged v0.3.0.854
** Invoke release_notes (first_time)
** Execute release_notes
** Invoke beta:archive (first_time)
** Invoke beta:build 
** Execute beta:archive
Archiving build...
rake aborted!
Unknown class NilClass. Try using :convert_unknown_to_string if you want to use unknown object types!
/Users/ben/.rvm/gems/ruby-1.9.2-p290@deli/gems/CFPropertyList-2.0.17/lib/rbCFPropertyList.rb:151:in `guess'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@deli/gems/CFPropertyList-2.0.17/lib/rbCFPropertyList.rb:125:in `block in guess'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@deli/gems/CFPropertyList-2.0.17/lib/rbCFPropertyList.rb:123:in `each_pair'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@deli/gems/CFPropertyList-2.0.17/lib/rbCFPropertyList.rb:123:in `guess'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@deli/gems/CFPropertyList-2.0.17/lib/rbCFPropertyList.rb:125:in `block in guess'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@deli/gems/CFPropertyList-2.0.17/lib/rbCFPropertyList.rb:123:in `each_pair'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@deli/gems/CFPropertyList-2.0.17/lib/rbCFPropertyList.rb:123:in `guess'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@deli/gems/CFPropertyList-2.0.17/lib/rbCFPropertyList.rb:371:in `to_plist'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@deli/bundler/gems/betabuilder-b8730fc0b482/lib/beta_builder/archived_build.rb:87:in `block in write_plist_to'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@deli/bundler/gems/betabuilder-b8730fc0b482/lib/beta_builder/archived_build.rb:86:in `open'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@deli/bundler/gems/betabuilder-b8730fc0b482/lib/beta_builder/archived_build.rb:86:in `write_plist_to'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@deli/bundler/gems/betabuilder-b8730fc0b482/lib/beta_builder/archived_build.rb:66:in `save_to'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@deli/bundler/gems/betabuilder-b8730fc0b482/lib/beta_builder.rb:159:in `block (2 levels) in define'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@global/gems/rake-0.9.2/lib/rake/task.rb:205:in `call'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@global/gems/rake-0.9.2/lib/rake/task.rb:205:in `block in execute'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@global/gems/rake-0.9.2/lib/rake/task.rb:200:in `each'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@global/gems/rake-0.9.2/lib/rake/task.rb:200:in `execute'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@global/gems/rake-0.9.2/lib/rake/task.rb:158:in `block in invoke_with_call_chain'
/Users/ben/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/monitor.rb:201:in `mon_synchronize'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@global/gems/rake-0.9.2/lib/rake/task.rb:151:in `invoke_with_call_chain'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@global/gems/rake-0.9.2/lib/rake/task.rb:176:in `block in invoke_prerequisites'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@global/gems/rake-0.9.2/lib/rake/task.rb:174:in `each'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@global/gems/rake-0.9.2/lib/rake/task.rb:174:in `invoke_prerequisites'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@global/gems/rake-0.9.2/lib/rake/task.rb:157:in `block in invoke_with_call_chain'
/Users/ben/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/monitor.rb:201:in `mon_synchronize'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@global/gems/rake-0.9.2/lib/rake/task.rb:151:in `invoke_with_call_chain'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@global/gems/rake-0.9.2/lib/rake/task.rb:144:in `invoke'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@global/gems/rake-0.9.2/lib/rake/application.rb:112:in `invoke_task'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@global/gems/rake-0.9.2/lib/rake/application.rb:90:in `block (2 levels) in top_level'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@global/gems/rake-0.9.2/lib/rake/application.rb:90:in `each'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@global/gems/rake-0.9.2/lib/rake/application.rb:90:in `block in top_level'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@global/gems/rake-0.9.2/lib/rake/application.rb:129:in `standard_exception_handling'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@global/gems/rake-0.9.2/lib/rake/application.rb:84:in `top_level'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@global/gems/rake-0.9.2/lib/rake/application.rb:62:in `block in run'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@global/gems/rake-0.9.2/lib/rake/application.rb:129:in `standard_exception_handling'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@global/gems/rake-0.9.2/lib/rake/application.rb:59:in `run'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@global/gems/rake-0.9.2/bin/rake:32:in `<top (required)>'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@global/bin/rake:19:in `load'
/Users/ben/.rvm/gems/ruby-1.9.2-p290@global/bin/rake:19:in `<main>'
Tasks: TOP => release => beta:archive

Here is the plist hash before the .to_plist call:

PLIST: {"ApplicationProperties"=>{"ApplicationPath"=>"Applications/deliapp.app.app", "CFBundleIdentifier"=>"com.chaione.deliapp", "CFBundleShortVersionString"=>nil, "IconPaths"=>["Applications/deliapp.app/Icon.png", "Applications/deliapp.app/Icon@2x.png", "Applications/deliapp.app/Icon-72.png", "Applications/deliapp.app/Icon-Small-50.png", "Applications/deliapp.app/Icon-Small.png", "Applications/deliapp.app/Icon-Small@2x.png"]}, "ArchiveVersion"=>1.0, "Comment"=>"07/27/2011 - RELEASE NOTES FOR VERSION v0.3.0.854 \n\n\n\n\n07/27/2011 - RELEASE NOTES FOR VERSION v0.3.0.852 \n\nFix for Bug #16263443\nFor for Bug #16225849\nUpdated graphics for the player bar, including an active/inactive speaker\n\n07/27/2011 - RELEASE NOTES FOR VERSION v0.3.0.842 \n\n- Re-enabled background audio. \n- Can now play music for a band.\n- Clear song details on player bar when app launches.  \n- Show player when a playlist is requested\n- Better handling of genres with the API\n- Updated genres to match server\n", "CreationDate"=>2011-07-27 13:06:37 -0500, "Name"=>"deliapp", "SchemeName"=>"deliapp"}