Object of class WP_Error could not be converted to string

This is the code we used in functions.php to register our custom post type for the REST API.

/**
* Add REST API support to an already registered post type.
*/
add_action( 'init', 'my_custom_post_type_rest_support', 25 );
function my_custom_post_type_rest_support() {
  global $wp_post_types;

  //be sure to set this to the name of your post type!
  $post_type_name = 'custom-post-type';
  if( isset( $wp_post_types[ $post_type_name ] ) ) {
    $wp_post_types[$post_type_name]->show_in_rest = true;
    $wp_post_types[$post_type_name]->rest_base = $post_type_name;
    $wp_post_types[$post_type_name]->rest_controller_class = 'WP_REST_Posts_Controller';
  }
}

Just to confirm, you’re using 2.0 Beta 11? If not, can you update to see if the error persists?

We updated to 2.0 Beta 11 but we are still receiving the error, just line no changed from 1226 to 1236

Here’s the corresponding code:

if ( $featured_media = get_post_thumbnail_id( $post->ID ) ) {
	$image_url = rest_url( 'wp/v2/media/' . $featured_media );
	$links['https://api.w.org/featuredmedia'] = array(
		'href'       => $image_url,
		'embeddable' => true,
	);
}

The fatal error is occurring on $image_url = rest_url( 'wp/v2/media/' . $featured_media );.

It appears you have a WP_Error object stored in the _thumbnail_id meta field for that post. I’d recommend taking a look at your application to see why this might be happening, and removing the object from the database.