WP_Query doesn't seem to filter with meta_query

I’ve added a loop to a template used for a single post, but WP_Query doesn’t seem to filter the posts collected according to their meta fields

I created two custom post types: “issue” and “article”.
“Issues” have post ids (obviously). And each “article” has a meta-field called “journal_article_in_issue” containing the post id of the issue to which they belong.

I’m trying to use WP_Query to collect all the posts with both the post-type of “article” and an “in-issue” value equal to the current issue post id.

This is the code intended to act as a loop within the single-issue template. It does collect all the posts with the post_type of ‘article’, but it is collecting all the posts regardless of the value of the ‘journal_article_in_issue’ meta field.

<?php
$issue_id=get_the_ID();
$post=get_post($issue_id);
?>

<!-- do some stuff with the single issue's information -->

<!-- display articles in issue -->
<div class="journal-issue-contents">
    <?php
        $article_query_args = array(
                    'post_type' => 'article',
                    'meta_query' => array(
                             'journal_article_in_issue' => $issue_id
                     )
        );
        $article_query = new WP_Query($article_query_args);
        if($article_query->have_posts()) {
            while($article_query->have_posts()) {
                $article_query->the_post();
                echo '<div>' . get_the_title() . '</div>';
            }
        }
    ?>
</div>

The example page is http://amosglenn.com/ssa/issue/january-june-2013/

Full template file is at http://pastebin.com/gX2D0X9r

Thanks!