get_terms – only top level

I am trying to get only top level term:

$cat_args = array(
    'parent '       => 0,
    'number'        => 10,
    'hide_empty'    => false,           
);

$categories = get_terms( 'question_category' , $cat_args); 

But this query return all childterms too, I tried everything but it always get child terms too.

I am trying since last 5 hours and can’t find whats wrong in my code, is this a WP bug or there is something wrong in my code ?

Thanks for helping.

Your code is correct, well almost correct. On first view, I must confess, I missed it too.

You have two syntax errors in your code. If you look closely, 'parent ' and 'parent' is not the same. You should not leave blank spaces between single quotes (') and arguments.

Also, you don’t need to add a , after your last argument.

This should work

$cat_args = array(
    'parent'        => 0,
    'number'        => 10,
    'hide_empty'    => false           
);

EDIT 15-06-2016

As from WordPress 4.5, the syntax has changed for get_terms(). The taxonomy parameter is now included in the array or args, so the new use will look like this

$args = [
    'taxonomy'     => 'my_tax',
    'parent'        => 0,
    'number'        => 10,
    'hide_empty'    => false           
];
$terms = get_terms( $args );