Using tax_query changes my post_type query in my WP_Query arguments

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

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