Unable to display multiple post types in same query (WPML WP_Query)

I had this problem with querying on two custom post types, using WP_Query. I had no problem querying for one type or the other in the array, but not both at the same time.

Did not work:

$args = array( 
    'post_type' => array('custom_type_1','custom_type_2'), 
    'posts_per_page' => 4
);

Did work:

$args = array( 
    'post_type' => array('custom_type_1'), 
    'posts_per_page' => 4
);

Did work:

$args = array( 
    'post_type' => array('custom_type_2'), 
    'posts_per_page' => 4
);

Awfully confusing to say the least.

Solved the problem by making sure that both custom post types had the “Make ‘Custom Post Type Name’ translatable” checkbox for WPML. WPML must not like it if you try to query on two post types where one is translatable and the other is not.

(I’m on WordPress 3.4.1 and WPML 2.5.2 – latest as of the date of this post)