diff --git a/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php b/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php
index 40955b3c1d..4c495041af 100644
--- a/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php
+++ b/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php
@@ -1923,10 +1923,16 @@ class WP_REST_Posts_Controller extends WP_REST_Controller {
 
 				case 'post-formats':
 					$supports_formats = get_theme_support( 'post-formats' );
+
+					// Force to an array. Supports formats can return true even if empty in some cases.
+					$supports_formats = is_array( $supports_formats ) ? array_values( $supports_formats[0] ) : array();
+
+					$supported_formats = array_merge( array( 'standard' ), $supports_formats );
+
 					$schema['properties']['format'] = array(
 						'description' => __( 'The format for the object.' ),
 						'type'        => 'string',
-						'enum'        => array_merge( array( 'standard' ), $supports_formats ? array_values( $supports_formats[0] ) : array() ),
+						'enum'        => $supported_formats,
 						'context'     => array( 'view', 'edit' ),
 					);
 					break;
diff --git a/wp-includes/version.php b/wp-includes/version.php
index b95578e02a..ece4f3b5dd 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.8-alpha-39619';
+$wp_version = '4.8-alpha-39620';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.