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The current error message is not really very descriptive if /check/referer is requested, but there's no Referer header (URI scheme "undefined" not supported). Should be easy to detect. cf. <http://lists.w3.org/Archives/Public/www-validator/2002Dec/0017.html>
This is in 0.6.1, updating after making 'zilla know about that version.
Fixed. We now give a /referer-specific error message for this case.