posts_per_page not working in get_posts

hi guys
i wrote this code for my template but i have problem.
here is my code:

<?php
$cat_id = get_cat_ID( single_cat_title("",false) );
global $post;
$args = array(
  'post_type' => 'post',
  'posts_per_page' => 6,
  'category' => $cat_id,
  'tax_query' => array(
    array(
      'taxonomy' => 'post_format',
      'field' => 'slug',
      'terms' => 'post-format-image',
      'operator' => 'NOT IN',
    ))
);
$myposts = get_posts( $args );
foreach( $myposts as $post ) :  setup_postdata($post);
  the_title( '<h4>', '</h4>' );
endforeach;
?>

unfortunately posts_per_page not working 🙁

[Moderator Note: No bumping, thank you.]