关联数组中的值数组在推入父数组时创建键

I want to add keys to this WP_Query $args array. They have nested arrays and for post-format the terms has to have an value array without keys to work. However when adding a key it autocreates a key '0' => 'value'.

PHP:

$args = array(
 'hierarchical' => true, 
 'posts_per_page' => $posts_per_page,
);

$post_formats_query = array(             
    'taxonomy' => 'post_format',
    'field' => 'slug',
    'terms' => array('post-format-'. $format_value);
);

$args['tax_query']=$post_formats_query; //this seems to create unwanted key

Var dump:

array (
  'hierarchical' => true,
  'posts_per_page' => '-1',
  'post_type' => 'post',
  'tax_query' => 
  array (
    'taxonomy' => 'post_format',
    'field' => 'slug',
    'terms' => 
    array (
      0 => 'post-format-gallery', //I WANT TO REMOVE THIS KEY
    ),
  ),
  'category_name' => NULL,
)