get_the_post_thumbnail() - Invalid argument supplied for foreach() in /wp-includes/post-thumbnail-template.php on line 64

Question

The past couple of hours I am trying to find out what is the problem with this...

<?php $related_posts = get_field('related_posts'); ?>
<?php if ($related_posts) : ?>
    <div class="related-projects_items-list-container">
        <div class="comments-area-title">
            <h4 class="hN"><?php _e('<em>Related</em> Articles', wpgrade::textdomain()); ?></h4>
        </div>
        <ul class="related-projects_items-list grid">
        <?php
        foreach ($related_posts as $post_object) {
            $the_post_ID = $post_object->ID;
            $the_permalink = get_permalink( $post_object->ID );
            $the_title = get_the_title( $post_object->ID );
            $the_thumbnail = get_the_post_thumbnail( $post_object->ID, array(200,150) );
            $the_post_author = $post_object->post_author;
            ?>
            <li class="related-projects_item grid__item one-half lap-and-up-one-quarter" style="margin-bottom: 20px;">
                 <article class="article article--billboard-small">
                    <a href="<?php echo $the_permalink; ?>">
                        <div class="article__thumb">
                            <?php if (has_post_thumbnail( $the_post_ID )) echo $the_thumbnail; ?>
                        </div>
                        <div class="article__content">
                            <h2 class="article__title article--billboard-small__title">
                                <span class="hN"><?php echo $the_title; ?></span>
                            </h2>
                            <span class="article__description">
                                <?php $author_url = get_author_posts_url( $the_post_author ); ?>
                                <?php $author_name = get_the_author_meta( 'display_name', $the_post_author ); ?>
                                <?php echo $author_name; ?> - <?php echo get_post_time('d.m.Y', false, $the_post_ID, false); ?>
                            </span>
                            <span class="small-link"><?php _e('Read More', wpgrade::textdomain()); ?><em>+</em></span>
                        </div> 
                    </a>
                </article>
            </li>
        <?php } ?>
        </ul>
    </div>
<?php endif; ?>

I get this...

Warning: Invalid argument supplied for foreach() in /wp-includes/post-thumbnail-template.php on line 64

The problem is this line...

$the_thumbnail = get_the_post_thumbnail( $post_object->ID, array(200,150) );

I can't understand why! Can someone please help?

I am using WordPress v.3.9.1

Answer 1

This Trac ticket here describes your problem and the cause:

https://core.trac.wordpress.org/ticket/26321

I got a warning/error related to update_post_thumbnail_cache() function:

Warning: Invalid argument supplied for foreach() in ./wp-includes/post-thumbnail-template.php on line 64 The $wp_query->posts array is missing, but we have the single $wp_query->post.

foreach ( $wp_query->posts as $post ) {
  if ( $id = get_post_thumbnail_id( $post->ID ) )
      $thumb_ids[] = $id;
}

Ticket #26321 describes that when a custom WP_Query loop on a page tries to grab a post thumbnail, Core tries to cache the thumbnails of the posts in the current query.

The problem happens because when doing so it loops over the posts in the query, but it never checks if the array of posts is empty or not. If it is, then you get the warning.

For the moment I would wait for a resolution in WP Core. The warning isn't breaking anything in your codes logic ( though a warning is still, undesirable ).

You can help though, log in to trac, download the patches and help test them. The more testing and help you can provide, the faster the bug can be fixed and the sooner it can get into WordPress Core.

If you manage to fix the bug yourself and satisfy all the unit tests, you might get props! Your name would be on the about/contributors page in every WordPress install running that release

Answer 2

Add a if statement, that you have a thumbnail, before you get the thumbnail.

if ( has_post_thumbnail( $post_object->ID ) )
     $the_thumbnail = get_the_post_thumbnail( $post_object->ID, array(200,150) );

Answer 3

I don't know why you think the post thumbnails are to blame, but the warning mentioned a foreach. If we remove the html we see this:

$related_posts = get_field('related_posts');
if ( $related_posts ) {
    foreach ( $related_posts as $post_object ) {

Clearly this is the cause of your warning.

Since you're returning a value for a field that has multiple options, this would be a better check:

$related_posts = get_field('related_posts');
if ( !empty( $related_posts ) ) {
    foreach ( $related_posts as $post_object ) {

This will now catch:

• empty arrays
• null
• empty strings
• false values