I'm a bit confused, what is the default value for the $size parameter?
/wp-includes/post-thumbnail-template.php
  52  /**
  53   * Retrieve Post Thumbnail.
  54   *
  55   * @since 2.9.0
  56   *
  57   * @param int $post_id Optional. Post ID.
  58   * @param string $size Optional. Image size.  Defaults to 'thumbnail'.
  59   * @param string|array $attr Optional. Query string or array of attributes.
  60    */
  61  function get_the_post_thumbnail( $post_id = NULL, $size = 'post-thumbnail', $attr =  ) {


I spotted this too. I'm gonna correct. I'm pretty sure post-thumbnail is wrong. I'll check the code first.