Index: src/wp-includes/query.php
===================================================================
--- src/wp-includes/query.php	(revision 27504)
+++ src/wp-includes/query.php	(working copy)
@@ -3663,7 +3663,11 @@
 					$term = get_term_by( 'slug', $this->get( 'category_name' ), 'category' );
 				}
 			} elseif ( $this->is_tag ) {
-				$term = get_term( $this->get( 'tag_id' ), 'post_tag' );
+				if ( $this->get( 'tag_id' ) ) {
+					$term = get_term( $this->get( 'tag_id' ), 'post_tag' );
+				} elseif ( $this->get( 'tag' ) ) {
+					$term = get_term_by( 'slug', $this->get( 'tag' ), 'post_tag' );
+				}
 			} else {
 				$tax_query_in_and = wp_list_filter( $this->tax_query->queries, array( 'operator' => 'NOT IN' ), 'NOT' );
 				$query = reset( $tax_query_in_and );