WP_Query with meta_query return results where zero expected

I’m using a WP_Query with a meta_query key to get results from a custom post type using the following code:

$args = array(
    'post_type' => 'werknemers',
    'meta_query' => array(
	'relation' => 'AND',
	array (
		'meta_key' => 'user',
		'meta_value' => $_POST['user']
	),
	array (
		'meta_key' => 'city',
		'meta_value' => $_POST['city']
	)
    )
);

$post = new WP_Query($args);
var_dump($post->found_posts);

This code works, and returns all posible results (INT of currently two results). The values entered through $_POST are incorrect so it should return zero (int) results.

I can’t figure out what’s wrong with my code.
The strange this is that when I’m using the following arguments it does return zero results (int)..

$args = array(
	'post_type' => 'werknemers',
	'meta_key' => 'user',
	'meta_value' => $_POST['user']
);

So I’ll guess the problem is somewhere in the relation or, but using the documentation I can’t figure out what’s wrong.. All the help is appreciated, thanks in forward!