diff -rup ../views/modules/taxonomy/views_plugin_argument_validate_taxonomy_term.inc ./modules/taxonomy/views_plugin_argument_validate_taxonomy_term.inc
--- ../views/modules/taxonomy/views_plugin_argument_validate_taxonomy_term.inc	2011-03-18 10:58:55.000000000 +1100
+++ ./modules/taxonomy/views_plugin_argument_validate_taxonomy_term.inc	2011-03-19 18:50:54.667053996 +1100
@@ -69,6 +69,11 @@ class views_plugin_argument_validate_tax
 
         return empty($vids) || !empty($vids[$result->vid]);
       case 'tids':
+        // An empty argument is not a term so doesn't pass.
+        if (empty($argument)) {
+          return FALSE;
+        }
+
         $tids = new stdClass();
         $tids->value = $argument;
         $tids = views_break_phrase($argument, $tids);