Excluding posts from a custom taxonomy

I need to query all posts from a custom post that doesn’t have ‘kit’ applied to the taxonomy ‘image-gallery’.

I am using the following code but it returns no results:

<?php
	$query = array(
		'post_type' => 'image-gallery',
		'posts_per_page' => -1,
		'tax_query' => array(
			array(
				'taxonomy' => 'image-gallery',
				'field' => 'slug',
				'terms' => 'kit',
				'operator' => 'NOT IN'
			)
		)
	)
?>

Can anyone see what might be wrong with it?