Index: wp-includes/query.php
===================================================================
--- wp-includes/query.php	(revision 16253)
+++ wp-includes/query.php	(working copy)
@@ -1572,7 +1572,7 @@
 		$q['tax_query'] = $tax_query;
 
 		foreach ( $q['tax_query'] as $query ) {
-			if ( 'IN' == $query['operator'] ) {
+			if ( isset( $query['operator'] ) && 'IN' == $query['operator'] ) {
 				switch ( $query['taxonomy'] ) {
 					case 'category':
 						$this->is_category = true;