Ticket #22956: 22956.3.diff

wp-includes/template.php

@@ -59 +59 @@
  * @return string
  */
 function get_archive_template() {
-        $post_types = get_query_var( 'post_type' );
+        $post_types = array_filter( (array) get_query_var( 'post_type' ) );
 
         $templates = array();
 
-        foreach ( (array) $post_types as $post_type )
+        if ( count( $post_types ) == 1 ) {
+                $post_type = reset( $post_types );
                 $templates[] = "archive-{$post_type}.php";
+        }
         $templates[] = 'archive.php';
 
         return get_query_template( 'archive', $templates );