Ticket #15978: 15978.2.patch

query.php

@@ -2081 +2081 @@
                 }
 
                 // Back-compat
-                if ( $this->is_category || $this->is_tag || $this->is_tax ) {
+                if ( !empty($this->tax_query->queries) ) {
                         $tax_query_in = wp_list_filter( $this->tax_query->queries, array( 'operator' => 'IN' ) );
                         if ( !empty( $tax_query_in ) ) {
                                 if ( !isset( $q['taxonomy'] ) ) {