EmailLogService ErrorException: Object of class WP_Error could not be converted to string

[Kilbourne]

The problem is here : https://github.com/yehudah/Post-SMTP/blob/dev/Postman/Postman-Email-Log/PostmanEmailLogService.php#L137-L148

If $post_id = wp_insert_post( $my_post, true ); return a WP_Error,then here is used as a string:
$this->logger->debug( sprintf( 'Saved message #%s to the database', $post_id ) );