diff --git a/wp-includes/rest-api/endpoints/class-wp-rest-controller.php b/wp-includes/rest-api/endpoints/class-wp-rest-controller.php
index 389262274f..6dc971517d 100644
--- a/wp-includes/rest-api/endpoints/class-wp-rest-controller.php
+++ b/wp-includes/rest-api/endpoints/class-wp-rest-controller.php
@@ -519,6 +519,16 @@ abstract class WP_REST_Controller {
 	public function get_fields_for_response( $request ) {
 		$schema = $this->get_item_schema();
 		$fields = isset( $schema['properties'] ) ? array_keys( $schema['properties'] ) : array();
+
+		$additional_fields = $this->get_additional_fields();
+		foreach ( $additional_fields as $field_name => $field_options ) {
+			// For back-compat, include any field with an empty schema
+			// because it won't be present in $this->get_item_schema().
+			if ( is_null( $field_options['schema'] ) ) {
+				$fields[] = $field_name;
+			}
+		}
+
 		if ( ! isset( $request['_fields'] ) ) {
 			return $fields;
 		}
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 74f6dd8723..30b4a4d53d 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.1-alpha-44253';
+$wp_version = '5.1-alpha-44254';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.