I fixed it doing like below:
$meta_value = //data from an API (e.g.: 19, 34, 1290 etc)
$meta_value_string = "". $meta_value;
$user_id = wp_insert_user( $new_user_data );
update_user_meta( $user_id, 'my_meta_key', $meta_value_string );
Now it works fine.