Add post in different category with wp_insert_post

Hi this code works fine with me ecxept it post in the default category
can any one solve my proplem please ?

$post_title='test';
$post_title=trim($post_title);
$post_date='';
$post_date_gmt='';
$post_author=1;
$post_status = 'publish';
$post_category = '8';
$post_content='test';
$post_ID = wp_insert_post(array(

'post_title' 	=> $post_title,
'post_content' 	=> $post_content,
'post_category' => $post_category,
'post_status' 	=> $post_status,
'post_author'   => $post_author,
'post_date'     => $post_date,
'post_date_gmt' => $post_date_gmt
));