Index: wp-includes/class-wp-theme.php
===================================================================
--- wp-includes/class-wp-theme.php	(revision 51096)
+++ wp-includes/class-wp-theme.php	(working copy)
@@ -1196,7 +1196,10 @@
 		$files = (array) self::scandir( $this->get_stylesheet_directory(), $type, $depth );
 
 		if ( $search_parent && $this->parent() ) {
-			$files += (array) self::scandir( $this->get_template_directory(), $type, $depth );
+			$parent_files = self::scandir( $this->get_template_directory(), $type, $depth );
+			if ( ! empty( $parent_files ) ) {
+				$files += (array) $parent_files;
+			}
 		}
 
 		return $files;