Index: wp-includes/query.php
===================================================================
--- wp-includes/query.php	(revision 23297)
+++ wp-includes/query.php	(working copy)
@@ -2256,25 +2256,30 @@
 				}
 
 				$cat_query = wp_list_filter( $tax_query_in_and, array( 'taxonomy' => 'category' ) );
-				if ( !empty( $cat_query ) ) {
+				if ( ! empty( $cat_query ) ) {
 					$cat_query = reset( $cat_query );
-					$the_cat = get_term_by( $cat_query['field'], $cat_query['terms'][0], 'category' );
-					if ( $the_cat ) {
-						$this->set( 'cat', $the_cat->term_id );
-						$this->set( 'category_name', $the_cat->slug );
+
+					if ( ! empty( $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 );
+							$this->set( 'category_name', $the_cat->slug );
+						}
+						unset( $the_cat );
 					}
-					unset( $the_cat );
 				}
 				unset( $cat_query );
 
 				$tag_query = wp_list_filter( $tax_query_in_and, array( 'taxonomy' => 'post_tag' ) );
-				if ( !empty( $tag_query ) ) {
+				if ( ! empty( $tag_query ) ) {
 					$tag_query = reset( $tag_query );
-					$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 );
+
+					if ( ! empty( $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 );
+						unset( $the_tag );
 					}
-					unset( $the_tag );
 				}
 				unset( $tag_query );
 			}