Using tax_query reverses my post_type argument in a custom WP_Query

I’m using a custom query to pull some posts from a custom post type. I’m having problems with it so I used print_r on my WP_Query object and I found something strange.

my query has these arguments:
'post_type' => 'event'
    ,'tax_query' => array(
                array(
                    'taxonomy' => 'event-categories'
                    ,'field' => 'id'
                    ,'terms' => 72
                    ,'operator' => 'IN'
                )
    )
    ,'order' => 'ASC'
);
$query = new WP_Query( $args );

and when I remove the tax_query portion I get all my posts, but when I include it I get none. I’m positive I have more than one post with the taxonomy ID of 72. When I look at my print_r($query) with the post type in, I see this:

WP_Query Object
(
[query_vars] => Array
    (
        [post_type] => Array
            (
                [0] => media_player
                [1] => ada_slides
                [2] => closings
                [3] => staff
                [4] => post
                [5] => page
            )

All of my post types are listed except for the one I wanted, event. If I take out tax_query from my args, it looks like this:

WP_Query Object
(
[query_vars] => Array
    (
        [post_type] => event
        [order] => ASC

…and so on

Also, if I change my operator to NOT IN, it searches the correct post type but excludes the category I want to search for.

Why would using a tax_query argument seemingly invalidate my post_type argument and instead include all post types but the one I wanted?