Issue with tax_query

Trying to delete all posts of a certain type (in this case ‘chart’) in a certain custom taxonomy (in this case ‘site’) that belong to a term when that term is deleted. Here’s what I have at the moment:

add_action( 'delete_term', 'remove_site', 10, 3 );

function remove_site( $term_id, $tt_id, $taxonomy ){
  if ($taxonomy != 'sites')
    return false;

  $args = array(
    'numberposts' => -1,
    'post_type' => 'chart',
    'tax_query' => array(
      array(
        'taxonomy' => 'sites',
        'field' => 'id',
        'terms'    => $term_id,
      ),
    ),
  );

  $posts = get_posts( $args );
  foreach ($posts as $post) {
    wp_delete_post( $post->ID, true );
  }
}

However, this does nothing. If I remove the tax_query (or make it a single level array), all posts of type ‘chart’ are deleted, so it must be something to do with the tax_query but I’m using the exact same query successfully elsewhere within a get_posts… I am, in fact using the exact same $args array elsewhere successfully, so I’m stumped.

I’ve checked that term_id is set properly, and it is (returning an integer) and $taxonomy is indeed ‘sites’.