Index: wp-includes/widgets.php
===================================================================
--- wp-includes/widgets.php	(revision 41339)
+++ wp-includes/widgets.php	(working copy)
@@ -1037,7 +1037,8 @@
 	global $wp_widget_factory;
 
 	if ( ! isset( $wp_widget_factory->widgets[ $widget ] ) ) {
-		_doing_it_wrong( __FUNCTION__, __( 'Widgets need to be registered before they can be displayed.' ), '4.9.0' );
+		/* translators: %s: register_widget() */
+		_doing_it_wrong( __FUNCTION__, sprintf( __( 'Widgets need to be registered using %s, before they can be displayed.' ), '<code>register_widget()</code>' ), '4.9.0' );
 		return;
 	}