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 f98d1ad57e..caee7204ea 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 d6ea670880..22ca5d3164 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.0-beta5-43907';
+$wp_version = '5.0-beta5-43908';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.