Tax Query on product_cat using NOT IN as operator does not exclude that category

Using the following code, I expect $items to not include products with the product category stones. However, that is not the result. It doesn't seem to be excluding that category at all. Syntax looks right - are there other reasons why WordPress wouldn't respect this NOT IN statement?

$tax_query = array(
    array(
        'taxonomy'         = 'product_cat',
        'terms'            = array('stones'),
        'field'            = 'slug',
        'operator'         = 'NOT IN',
    ),
);
$args = array(
            'post_type' = 'product',
            'posts_per_page' = -1,
            'tax_query' = $tax_query
        );
$items = new WP_Query( $args );