WP Query – Post Thumbnail

I am using the plugin: Series and by default it only gets the linked posts titles. I would like to add the post thumbnails as well. I was able to get the thumbnails to appear, but they show up outside of the widget. I added: the_post_thumbnail(‘thumbnail’) parts. The full template is below.

Thank you for the help!

/**
 * Displays a list of posts by series ID.
 *
 * @since  0.1.0
 * @param  array   $args
 * @return string
 */
function series_list_posts( $args = array() ) {

    if ( empty( $args['series'] ) )
        return;

    $out     = '';
    $post_id = 0;

    if ( in_the_loop() )
        $post_id = get_the_ID();

    else if ( is_singular() )
        $post_id = get_queried_object_id();

    $defaults = array(
        'series'         => '', // term slug
        'order'          => 'ASC',
        'orderby'        => 'date',
        'posts_per_page' => -1,
        'echo'           => true,
    );

    $args = wp_parse_args( $args, $defaults );

    $loop = new WP_Query( $args );

    if ( $loop->have_posts() ) {

        $out .= '<ul class="series-list">';

        while ( $loop->have_posts() ) {

            $loop->the_post();

            $out .= $post_id === get_the_ID() ? the_title( '<li>' . the_post_thumbnail('thumbnail') .  '</li>', false ) : the_title( '<li>' . the_post_thumbnail('thumbnail') . '<a href="' . get_permalink() . '">', '</a></li>', false );
        }

        $out .= '</ul>';
    }

    wp_reset_postdata();

    if ( false === $args['echo'] )
        return $out;

    echo $out;
}

/**
 * Displays a list of posts related to the post by the first series.
 *
 * @since  0.1.0
 * @param  array  $args
 * @return string
 */
function series_list_related( $args = array() ) {

    $post_id = 0;

    if ( in_the_loop() )
        $post_id = get_the_ID();

    else if ( is_singular() )
        $post_id = get_queried_object_id();

    if ( !empty( $post_id ) )
        $series = get_the_terms( $post_id, 'series' );

    if ( empty( $series ) )
        return;

    $series = reset( $series );

    $args['series'] = $series->slug;

    return series_list_posts( $args );
}

/* === DEPRECATED === */

/**
 * @since      0.1.0
 * @deprecated 0.2.0
 */
function get_series_feed_link( $cat_id, $feed = '' ) {
    _deprecated_function( __FUNCTION__, '0.2.0', 'get_term_feed_link' );
    return get_term_feed_link( $term_id, 'series', $feed );
}

/**
 * @since      0.1.0
 * @deprecated 0.2.0
 */
function is_series( $slug = false ) {
    _deprecated_function( __FUNCTION__, '0.2.0', 'is_tax' );
    return is_tax( 'series', $slug );
}

/**
 * @since      0.1.0
 * @deprecated 0.2.0
 */
function in_series( $series, $_post = null ) {
    _deprecated_function( __FUNCTION__, '0.2.0', 'has_term' );
    return has_term( $series, 'series', $_post );
}

/**
 * @since      0.1.0
 * @deprecated 0.2.0
 */
function create_series_taxonomy() {
    _deprecated_function( __FUNCTION__, '0.2.0', '' );
}

/**
 * @since      0.1.0
 * @deprecated 0.2.0
 */
function series_register_widgets() {
    _deprecated_function( __FUNCTION__, '0.2.0', '' );
}