Ticket #40580: 40580.patch

src/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php

@@ -474 +474 @@
                 $data     = $this->prepare_item_for_response( $post, $request );
                 $response = rest_ensure_response( $data );
 
-                if ( is_post_type_viewable( get_post_type_object( $post->post_type ) ) ) {
+                if ( ! is_wp_error( $response ) && is_post_type_viewable( get_post_type_object( $post->post_type ) ) ) {
                         $response->link_header( 'alternate',  get_permalink( $post->ID ), array( 'type' => 'text/html' ) );
                 }