WP_Query with custom post type and default categories

I have a site with several custom post types.

I’ve set all my post types to use the same set of categories, with the following line in the registration $args:

'taxonomies' => array( 'category', 'post_tag' ),

That part works fine.

But when I try to do a query based on post_type and category, I get all results for that category, regardless of post_type.

For instance, here’s a shortcode that should only display results from the custom post type “emc-album’:

/* Photo albums */
function emc_photo_albums($atts, $content = null) {
	    $a = shortcode_atts( array(
        'number' => '',
        'albumcat' => ''
    ), $atts );

	$albumCode = '<h2>Photo albums | <a class="see-all" href="'.get_home_url().'/emc-album/">See all photos</a></h2><ul id="album_covers" class="list-unstyled">';

	$album_args = 'post_type=emc-album&number='.$a["number"].'&category_name='.$a["albumcat"].'&orderby=date';

	$album_query = new WP_Query($album_args);

	if ($album_query->have_posts()) : while ($album_query->have_posts()) : $album_query->the_post();

    		$albumCode .= '<li class="album-grid"><a class="clickable-div" href="'.get_permalink().'" title="'.get_the_title().'">&nbsp;</a>'.get_the_post_thumbnail($post->ID,"album-thumb").'<div class="album-title">'.get_the_title().'</div></li>';

    	endwhile;
    endif;

	$albumCode .= '<li class="clearfix"></li></ul>';
	wp_reset_postdata();
	return $albumCode;
	die();
}
add_shortcode('emc-photo-albums','emc_photo_albums');

The shortcode itself looks like this:

[emc-photo-albums albumcat="membership"]

If I don’t set a value for albumcat, I get what I expect: all posts of the “emc-album” type.

But if I set a value for albumcat, I get all posts with that category, regardless of post-type.

Any help would be appreciated!