Index: src/wp-includes/class-wp-editor.php
===================================================================
--- src/wp-includes/class-wp-editor.php	(revision 39643)
+++ src/wp-includes/class-wp-editor.php	(working copy)
@@ -681,8 +681,10 @@
 						$body_class .= ' post-format-standard';
 				}
 
-				if ( $page_template = get_page_template_slug( $post ) ) {
-					$page_template = str_replace( '.', '-', basename( $page_template, '.php' ) );
+				$page_template = get_page_template_slug( $post );
+
+				if ( $page_template !== false ) {
+					$page_template = $page_template === '' ? 'default' : str_replace( '.', '-', basename( $page_template, '.php' ) );
 					$body_class .= ' page-template-' . sanitize_html_class( $page_template );
 				}
 			}