diff --git src/wp-includes/widgets.php src/wp-includes/widgets.php
index 40bf54c..fa7790b 100644
--- src/wp-includes/widgets.php
+++ src/wp-includes/widgets.php
@@ -803,6 +803,11 @@ function register_sidebar($args = array()) {
 
 	$i = count($wp_registered_sidebars) + 1;
 
+	// Ensure we have a valid ID.
+	if ( empty( $args['id'] ) ) {
+		_doing_it_wrong( __METHOD__, __( 'The id argument should not be empty.' ), '4.2.0' );
+	}
+
 	$defaults = array(
 		'name' => sprintf(__('Sidebar %d'), $i ),
 		'id' => "sidebar-$i",