Query args category__in & tag__in combination doesn’t work

Hi,

I am trying to retrieve all posts from tag ID 6 in category ID 3.
For some reason, the below code only works with the combinations:
-category__in & tag__and
-category__and & tag__in
But it doesn’t work with category__in & tag__in:
$args=array(
‘category__in’ => array(3), ‘tag__in’ => array(6),
‘post__not_in’ => array($post->ID),
‘showposts’=>3,
‘caller_get_posts’=>1
);
$my_query = new WP_Query($args);

Please, can someone explain?