diff --git a/src/wp-includes/rest-api/endpoints/class-wp-rest-widget-types-controller.php b/src/wp-includes/rest-api/endpoints/class-wp-rest-widget-types-controller.php
index 1868050573..7862ad1d79 100644
--- a/src/wp-includes/rest-api/endpoints/class-wp-rest-widget-types-controller.php
+++ b/src/wp-includes/rest-api/endpoints/class-wp-rest-widget-types-controller.php
@@ -85,9 +85,9 @@ class WP_REST_Widget_Types_Controller extends WP_REST_Controller {
 					'form_data' => array(
 						'description'       => __( 'Serialized widget form data to encode into instance settings.' ),
 						'type'              => 'string',
-						'sanitize_callback' => static function( $string ) {
+						'sanitize_callback' => static function( $form_data ) {
 							$array = array();
-							wp_parse_str( $string, $array );
+							wp_parse_str( $form_data, $array );
 							return $array;
 						},
 					),