Index: wp-includes/query.php
===================================================================
--- wp-includes/query.php	(revision 17389)
+++ wp-includes/query.php	(working copy)
@@ -2181,6 +2181,8 @@
 				$cat_query = wp_list_filter( $tax_query_in_and, array( 'taxonomy' => 'category' ) );
 				if ( !empty( $cat_query ) ) {
 					$cat_query = reset( $cat_query );
+					if ( !isset( $cat_query['terms'][0] ))
+						$cat_query['terms'][0] = '';
 					$the_cat = get_term_by( $cat_query['field'], $cat_query['terms'][0], 'category' );
 					if ( $the_cat ) {
 						$this->set( 'cat', $the_cat->term_id );
@@ -2193,6 +2195,8 @@
 				$tag_query = wp_list_filter( $tax_query_in_and, array( 'taxonomy' => 'post_tag' ) );
 				if ( !empty( $tag_query ) ) {
 					$tag_query = reset( $tag_query );
+					if ( !isset( $tag_query['terms'][0] ))
+						$tag_query['terms'][0] = '';
 					$the_tag = get_term_by( $tag_query['field'], $tag_query['terms'][0], 'post_tag' );
 					if ( $the_tag ) {
 						$this->set( 'tag_id', $the_tag->term_id );