Problem executing sample code

I've installed the http connector and the gem as documented, but as soon as I run a bit of the sample code, I get an error:

>> require 'quickbooks'
=> true
>> Quickbooks.use_adapter :http, "qbserver","appname","passwd"
=> Quickbooks::Adapters::HttpAdapter::Connection
>> customers = QB::Customer.all
NameError: undefined method `attributes' for class `String'
    from /Library/Ruby/Gems/1.8/gems/quickbooks-0.9.9/lib/quickbooks/type.rb:72:in `method'
    from /Library/Ruby/Gems/1.8/gems/quickbooks-0.9.9/lib/quickbooks/type.rb:72:in `method_missing'
    from /Library/Ruby/Gems/1.8/gems/quickbooks-0.9.9/lib/quickbooks/property.rb:65:in `send'
    from /Library/Ruby/Gems/1.8/gems/quickbooks-0.9.9/lib/quickbooks/property.rb:65:in `method_missing'
    from /Library/Ruby/Gems/1.8/gems/quickbooks-0.9.9/lib/quickbooks/ruby_ext.rb:104:in `parse_xml'
    from /Library/Ruby/Gems/1.8/gems/quickbooks-0.9.9/lib/quickbooks.rb:108:in `send_request'
    from /Library/Ruby/Gems/1.8/gems/quickbooks-0.9.9/lib/quickbooks.rb:127:in `execute'
    from /Library/Ruby/Gems/1.8/gems/quickbooks-0.9.9/lib/quickbooks/model.rb:130:in `all'
    from /Library/Ruby/Gems/1.8/gems/quickbooks-0.9.9/lib/quickbooks/model.rb:129:in `catch'
    from /Library/Ruby/Gems/1.8/gems/quickbooks-0.9.9/lib/quickbooks/model.rb:129:in `all'
    from (irb):3

What am I missing?