Google Analytics _setCustomVar issue

I'm trying to set a custom variable in my Google Analytics tracking for a very rough form of split testing.

The code I'm using at the end of my 'head' section is

var _gaq = _gaq || [];
            _gaq.push(
                ['_setAccount', 'UA-xxxxxx-xx'],
                ['_setCustomVar', 1, 'split', '1', 2],
                ['_trackPageview']
            );
            (function() {
                var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
                ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
                var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);
            开发者_如何学运维})();

The page view is tracking fine but I'm not seeing anything other than "(not set)" under the User-defined sub-section of the Visitors section.

Can anyone spot anything wrong? I've had this working before using the older ga.js before the Asynchronous stuff came along.

Thanks.

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_setCustomVar data shows up in the Visitors "Custom Variables" report. Only data from _setVar shows up in the "User Defined" report.